Inverted metamorphic multijunction solar cell with metamorphic layers

ABSTRACT

A multijunction solar cell having at least four solar subcells includes a first solar subcell having a first band gap. A first graded interlayer adjacent to the first solar subcell and has a second band gap greater than the first band gap and that is constant at 1.5 eV throughout the thickness of the first graded interlayer. A second solar subcell is adjacent to the first graded interlayer and has a third band gap smaller than the first band gap of the first solar subcell. The second solar subcell is lattice mismatched with respect to the first solar subcell. A second graded interlayer is adjacent to the second solar subcell and has a fourth band gap greater than the third band gap of the second solar subcell and that is constant at 1.1 eV throughout the thickness of the second graded interlayer. A third solar subcell is adjacent to the second graded interlayer and has a fifth band gap smaller than the third band gap of the second solar subcell. The third solar subcell is lattice mismatched with respect to the second solar subcell. Each of the first and second graded interlayers is composed, respectively, of a compositionally step-graded series of (In x Ga 1-x ) y Al 1-y As layers with monotonically changing lattice constant, with x and y having respective values such that the band gap of each interlayer remains constant throughout its thickness, and wherein 0&lt;x&lt;1 and 0&lt;y&lt;1.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 12/271,192 filed Nov. 14, 2008, and of co-pendingU.S. patent application Ser. No. 12/023,772, filed Jan. 31, 2008, whichis in turn a continuation-in-part of co-pending U.S. patent applicationSer. No. 11/860,142 filed Sep. 24, 2007, and of co-pending U.S. patentapplication Ser. No. 11/860,183, filed Sep. 24, 2007.

This application is related to co-pending U.S. patent application Ser.No. 12/844,673 filed Jul. 27, 2010.

This application is related to co-pending U.S. patent application Ser.No. 12/813,408 filed Jun. 10, 2010.

This application is related to co-pending U.S. patent application Ser.No. 12/775,946 filed May 7, 2010.

This application is related to co-pending U.S. patent application Ser.No. 12/756,926, filed Apr. 8, 2010.

This application is related to co-pending U.S. patent application Ser.No. 12/730,018, filed Mar. 23, 2010.

This application is related to co-pending U.S. patent application Ser.No. 12/716,814, filed Mar. 3, 2010.

This application is related to co-pending U.S. patent application Ser.No. 12/708,361, filed Feb. 18, 2010.

This application is related to co-pending U.S. patent application Ser.No. 12/637,241, filed Dec. 14, 2009.

This application is related to co-pending U.S. patent application Ser.No. 12/623,134, filed Nov. 20, 2009.

This application is related to co-pending U.S. patent application Ser.No. 12/544,001, filed Aug. 19, 2009.

This application is related to co-pending U.S. patent application Ser.Nos. 12/401,137, 12/401,157, and 12/401,189, filed Mar. 10, 2009.

This application is related to co-pending U.S. patent application Ser.No. 12/389,053, filed Feb. 19, 2009.

This application is related to co-pending U.S. patent application Ser.No. 12/367,991, filed Feb. 9, 2009.

This application is related to U.S. patent application Ser. No.12/362,201, now U.S. Pat. No. 7,960,201; Ser. Nos. 12/362,213; and12/362,225, filed Jan. 29, 2009.

This application is related to U.S. patent application Ser. No.12/337,014 filed Dec. 17, 2008, now U.S. Pat. No. 7,785,989, and Ser.No. 12/337,043 filed Dec. 17, 2008.

This application is related to co-pending U.S. patent application Ser.Nos. 12/271,127 and 12/271,192 filed Nov. 14, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/267,812 filed Nov. 10, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/258,190 filed Oct. 24, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/253,051 filed Oct. 16, 2008.

This application is related to U.S. patent application Ser. No.12/190,449, filed Aug. 12, 2008, now U.S. Pat. No. 7,741,146, and itsdivisional patent application Ser. No. 12/816,205, filed Jun. 15, 2010,now U.S. Pat. No. 8,039,291.

This application is related to co-pending U.S. patent application Ser.No. 12/187,477, filed Aug. 7, 2008.

This application is related to co-pending U.S. patent application Ser.Nos. 12/218,558 and 12/218,582 filed Jul. 16, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/123,864 filed May 20, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/102,550 filed Apr. 14, 2008.

This application is related to co-pending U.S. Ser. No. 12/047,944,filed Mar. 13, 2008.

This application is related to co-pending U.S. patent application Ser.No. 12/023,772, filed Jan. 31, 2008.

This application is related to U.S. patent application Ser. No.11/956,069, filed Dec. 13, 2007, and its divisional application Ser. No.12/187,454 filed Aug. 7, 2008, now U.S. Pat. No. 7,727,795;

This application is also related to co-pending U.S. patent applicationSer. Nos. 11/860,142 and 11/860,183 filed Sep. 24, 2007.

This application is also related to co-pending U.S. patent applicationNo. 11/445,793 filed Jun. 2, 2006.

This application is also related to co-pending U.S. patent applicationSer. No. 11/500,053 filed Aug. 7, 2006, and its divisional applicationsSer. No. 12/417,367 filed Apr. 2, 2009, and Ser. No. 12/549,340 filedAug. 27, 2009.

GOVERNMENT RIGHTS STATEMENT

This invention was made with government support under Contracts No. FA9453-06-C-0345, FA9453-09-C-0371 and FA 9453-04-2-0041 awarded by theU.S. Air Force. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of multijunction solar cellsbased on III-V semiconductor compounds, and to fabrication processes anddevices for five and six junction solar cell structures including ametamorphic layer. Some embodiments of such devices are also known asinverted metamorphic multijunction solar cells.

2. Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology not only for use in spacebut also for terrestrial solar power applications. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to manufacture. Typical commercialIII-V compound semiconductor multijunction solar cells have energyefficiencies that exceed 27% under one sun, air mass 0 (AM0),illumination, whereas even the most efficient silicon technologiesgenerally reach only about 18% efficiency under comparable conditions.Under high solar concentration (e.g., 500×), commercially availableIII-V compound semiconductor multijunction solar cells in terrestrialapplications (at AM1.5D) have energy efficiencies that exceed 37%. Thehigher conversion efficiency of III-V compound semiconductor solar cellscompared to silicon solar cells is in part based on the ability toachieve spectral splitting of the incident radiation through the use ofa plurality of photovoltaic regions with different band gap energies,and accumulating the current from each of the regions.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures. Theindividual solar cells or wafers are then disposed in horizontal arrays,with the individual solar cells connected together in an electricalseries circuit. The shape and structure of an array, as well as thenumber of cells it contains, are determined in part by the desiredoutput voltage and current.

Inverted metamorphic solar cell structures based on III-V compoundsemiconductor layers, such as described in M. W. Wanlass et al., LatticeMismatched Approaches for High Performance, III-V Photovoltaic EnergyConverters (Conference Proceedings of the 31^(st) IEEE PhotovoltaicSpecialists Conference, Jan. 3-7, 2005, IEEE Press, 2005), present animportant conceptual starting point for the development of futurecommercial high efficiency solar cells. However, the materials andstructures for a number of different layers of the cell proposed anddescribed in such reference present a number of practical difficultiesrelating to the appropriate choice of materials and fabrication steps.

Prior to the disclosures described in various ones or combinations ofthis and the related applications noted above, the materials andfabrication steps disclosed in the prior art have various drawbacks anddisadvantages in producing a commercially viable inverted metamorphicmultijunction solar cell using commercially established fabricationprocesses.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present disclosure provides a fivejunction solar cell utilizing two metamorphic layers. More particularlythe present disclosure provides a multijunction solar cell including anupper first solar subcell having a first band gap, and the base-emitterjunction of the upper first solar subcell being a homojunction; a secondsolar subcell adjacent to said first solar subcell and having a secondband gap smaller than said first band gap; and a third solar subcelladjacent to said second solar subcell and having a third band gapsmaller than said second band gap. A first graded interlayer is providedadjacent to said third solar subcell; said first graded interlayerhaving a fourth band gap greater than said third band gap. A fourthsolar subcell is provided adjacent to said first graded interlayer, saidfourth subcell having a fifth band gap smaller than said third band gapsuch that said fourth subcell is lattice mismatched with respect to saidthird subcell. A second graded interlayer is provided adjacent to saidfourth solar subcell; said second graded interlayer having a sixth bandgap greater than said fifth band gap; and a lower fifth solar subcell isprovided adjacent to said second graded interlayer, said lower fifthsubcell having a seventh band gap smaller than said fifth band gap suchthat said fifth subcell is lattice mismatched with respect to saidfourth subcell.

In another aspect, the present disclosure provides a five junction solarcell utilizing three metamorphic layers. More particularly the presentdisclosure provides a multijunction solar cell including an upper firstsolar subcell having a first band gap, and the base-emitter junction ofthe upper first solar subcell being a homojunction; a second solarsubcell adjacent to said first solar subcell and having a second bandgap smaller than said first band gap. A first graded interlayer isprovided adjacent to said second solar subcell; said first gradedinterlayer having a third band gap greater than said second band gap. Athird solar subcell is provided adjacent to said first graded interlayerand having a fourth band gap smaller than said second band gap such thatsaid third subcell is lattice mismatched with respect to said secondsubcell. A second graded interlayer is provided adjacent to said thirdsolar subcell; said second graded interlayer having a fifth band gapgreater than said fourth band gap. A fourth solar subcell is providedadjacent to said second graded interlayer, said fourth subcell having asixth band gap smaller than said fourth band gap such that said fourthsubcell is lattice mismatched with respect to said third subcell. Athird graded interlayer is provided adjacent to said fourth solarsubcell; said third graded interlayer having a seventh band gap greaterthan said sixth band gap. A lower fifth solar subcell is providedadjacent to said third graded interlayer, said lower fifth subcellhaving a eighth band gap smaller than said seventh band gap such thatsaid fifth subcell is lattice mismatched with respect to said fourthsubcell.

In another aspect the present disclosure provides a six junction solarcell utilizing three metamorphic layers. More particularly the presentdisclosure provides a multijunction solar cell including an upper firstsolar subcell having a first band gap, and the base-emitter junction ofthe upper first solar subcell being a homojunction; a second solarsubcell adjacent to said first solar subcell and having a second bandgap smaller than said first band gap; a third solar subcell adjacent tosaid second solar subcell and having a third band gap smaller than saidsecond band gap; a first graded interlayer adjacent to said third solarsubcell; said first graded interlayer having a fourth band gap greaterthan said third band gap; and a fourth solar subcell adjacent to saidfirst graded interlayer, said fourth subcell having a fifth band gapsmaller than said third band gap such that said fourth subcell islattice mismatched with respect to said third subcell; a second gradedinterlayer adjacent to said fourth solar subcell; said second gradedinterlayer having a sixth band gap greater than said fifth band gap; afifth solar subcell adjacent to said second graded interlayer, saidfifth subcell having a seventh band gap smaller than said fifth band gapsuch that said fifth subcell is lattice mismatched with respect to saidfourth subcell; a third graded interlayer adjacent to said fifth solarsubcell; said third graded interlayer having a eighth band gap greaterthan said seventh band gap; and a lower sixth solar subcell adjacent tosaid third graded interlayer, said lower sixth subcell having a ninthband gap smaller than said eighth band gap such that said sixth subcellis lattice mismatched with respect to said fifth subcell.

In another aspect the present disclosure provides a five junction solarcell utilizing one metamorphic layer. More particularly the presentdisclosure provides a multijunction solar cell including an upper firstsolar subcell having a first band gap, and the base-emitter junction ofthe upper first solar subcell being a homojunction; a second solarsubcell adjacent to said first solar subcell and having a second bandgap smaller than said first band gap; and a third solar subcell adjacentto said second solar subcell and having a third band gap smaller thansaid second band gap. A graded interlayer is provided adjacent to saidthird solar subcell; said graded interlayer having a fourth band gapgreater than said third band gap. A fourth solar subcell is providedadjacent to said first graded interlayer, said fourth subcell having afifth band gap smaller than said third band gap such that said fourthsubcell is lattice mismatched with respect to said third subcell. Alower fifth solar subcell is provided adjacent to said fourth subcell,said lower fifth subcell having a sixth band gap smaller than said fifthband gap such that said fourth subcell is lattice matched with respectto said fourth subcell.

In another aspect the present disclosure provides a six junction solarcell utilizing two metamorphic layers. More particularly the presentdisclosure provides a multijunction solar cell including an upper firstsolar subcell having a first band gap, and the base-emitter junction ofthe upper first solar subcell being a homojunction; a second solarsubcell adjacent to said first solar subcell and having a second bandgap smaller than said first band gap; and a third solar subcell adjacentto said second solar subcell and having a third band gap smaller thansaid second band gap. A first graded interlayer is provided adjacent tosaid third solar subcell; said first graded interlayer having a fourthband gap greater than said third band gap. A fourth solar subcell isprovided adjacent to said first graded interlayer, said fourth subcellhaving a fifth band gap smaller than said third band gap such that saidfourth subcell is lattice mismatched with respect to said third subcell.A fifth solar subcell is provided adjacent to said fourth subcell, saidfifth subcell having a sixth band gap smaller than said fifth band gapsuch that said fifth subcell is lattice matched with respect to saidfourth subcell. A second graded interlayer is provided adjacent to saidfifth solar subcell; said second graded interlayer having a seventh bandgap greater than said sixth band gap. A lower sixth solar subcell isprovided adjacent to said second graded interlayer, said lower sixthsubcell having a eighth band gap smaller than said sixth band gap suchthat said sixth subcell is lattice mismatched with respect to said fifthsubcell.

In another aspect the present disclosure provides a method of forming afive junction solar cell utilizing two metamorphic layers. Moreparticularly the present disclosure provides a method of manufacturing asolar cell including providing a first substrate; forming an upper firstsolar subcell having a first band gap on the first substrate, thebase-emitter junction of the upper first solar subcell being ahomojunction; forming a second solar subcell adjacent to said firstsolar subcell and having a second band gap smaller than said first bandgap; and forming a third solar subcell adjacent to said second solarsubcell and having a third band gap smaller than said second band gap. Afirst graded interlayer is formed adjacent to said third solar subcell;said first graded interlayer having a fourth band gap greater than saidthird band gap. A fourth solar subcell is foamed adjacent to said firstgraded interlayer, said fourth subcell having a fifth band gap smallerthan said third band gap such that said fourth subcell is latticemismatched with respect to said third subcell. A second gradedinterlayer is formed adjacent to said fourth solar subcell; said secondgraded interlayer having a sixth band gap greater than said fifth bandgap; and a lower fifth solar subcell is formed adjacent to said secondgraded interlayer, said lower fifth subcell having a seventh band gapsmaller than said fifth band gap such that said fifth subcell is latticemismatched with respect to said fourth subcell. A surrogate substrate ismounted on top of lower fifth solar subcell; and the first substrate isremoved.

In another aspect, the present disclosure provides a method of forming afive junction solar cell utilizing three metamorphic layers. Moreparticularly the present disclosure provides a method of manufacturing amultijunction solar cell including providing a first substrate; formingan upper first solar subcell having a first band gap, the base-emitterjunction of the upper first solar subcell being a homojunction; forminga second solar subcell adjacent to said first solar subcell and having asecond band gap smaller than said first band gap. A first gradedinterlayer is fowled adjacent to said second solar subcell; said firstgraded interlayer having a third band gap greater than said second bandgap. A third solar subcell is formed adjacent to said first gradedinterlayer having a fourth band gap smaller than said second band gapsuch that said third subcell is lattice mismatched with respect to saidsecond subcell. A second graded interlayer is formed adjacent to saidthird solar subcell; said second graded interlayer having a fifth bandgap greater than said fourth band gap. A fourth solar subcell is formedadjacent to said second graded interlayer, said fourth subcell having asixth band gap smaller than said fourth band gap such that said fourthsubcell is lattice mismatched with respect to said third subcell. Athird graded interlayer is formed adjacent to said fourth solar subcell;said third graded interlayer having a seventh band gap greater than saidsixth band gap. A lower fifth solar subcell is formed adjacent to saidthird graded interlayer, said lower fifth subcell having a eighth bandgap smaller than said seventh band gap such that said fifth subcell islattice mismatched with respect to said fourth subcell.

In another aspect the present disclosure provides a method of forming asix junction solar cell utilizing three metamorphic layers. Moreparticularly the present disclosure provides a method of manufacturing amultijunction solar cell including providing a first substrate; formingan upper first solar subcell having a first band gap on the firstsubstrate, the base-emitter junction of the upper first solar subcellbeing a homojunction; forming a second solar subcell adjacent to saidfirst solar subcell and having a second band gap smaller than said firstband gap; forming a third solar subcell adjacent to said second solarsubcell and having a third band gap smaller than said second band gap;forming a first graded interlayer adjacent to said third solar subcell;said first graded interlayer having a fourth band gap greater than saidthird band gap; forming a fourth solar subcell adjacent to said firstgraded interlayer, said fourth subcell having a fifth band gap smallerthan said third band gap such that said fourth subcell is latticemismatched with respect to said third subcell; forming a second gradedinterlayer adjacent to said fourth solar subcell; said second gradedinterlayer having a sixth band gap greater than said fifth band gap;forming a fifth solar subcell adjacent to said second graded interlayer,said fifth subcell having a seventh band gap smaller than said fifthband gap such that said fifth subcell is lattice mismatched with respectto said fourth subcell; forming a third graded interlayer adjacent tosaid fifth solar subcell; said third graded interlayer having a eighthband gap greater than said seventh band gap; and forming a lower sixthsolar subcell adjacent to said third graded interlayer, said lower sixthsubcell having a ninth band gap smaller than said eighth band gap suchthat said sixth subcell is lattice mismatched with respect to said fifthsubcell. A surrogate substrate is mounted on top of lower sixth solarsubcell; and the first substrate is removed.

In another aspect the present disclosure provides a method of forming afive junction solar cell utilizing one metamorphic layer. Moreparticularly the present disclosure provides a method of manufacturing amultijunction solar cell including providing a first substrate; formingan upper first solar subcell having a first band gap, and thebase-emitter junction of the upper first solar subcell being ahomojunction; forming a second solar subcell adjacent to said firstsolar subcell and having a second band gap smaller than said first bandgap; and forming a third solar subcell adjacent to said second solarsubcell and having a third band gap smaller than said second band gap. Agraded interlayer is formed adjacent to said third solar subcell; saidgraded interlayer having a fourth band gap greater than said third bandgap. A fourth solar subcell is formed adjacent to said first gradedinterlayer, said fourth subcell having a fifth band gap smaller thansaid third band gap such that said fourth subcell is lattice mismatchedwith respect to said third subcell. A lower fifth solar subcell isformed adjacent to said fourth subcell, said lower fifth subcell havinga sixth band gap smaller than said fifth band gap such that said fourthsubcell is lattice matched with respect to said fourth subcell.

In another aspect the present disclosure provides a method of forming asix junction solar cell utilizing two metamorphic layers. Moreparticularly the present disclosure provides a method of manufacturing amultijunction solar cell including providing a first substrate; formingan upper first solar subcell being a homojunction; forming a secondsolar subcell adjacent to said first solar subcell and having a secondband gap smaller than said first band gap; and forming a third solarsubcell adjacent to said second solar subcell and having a third bandgap smaller than said second band gap. A first graded interlayer isformed adjacent to said third solar subcell; said first gradedinterlayer having a fourth band gap greater than said third band gap. Afourth solar subcell is formed adjacent to said first graded interlayer,said fourth subcell having a fifth band gap smaller than said third bandgap such that said fourth subcell is lattice mismatched with respect tosaid third subcell. A fifth solar subcell is formed adjacent to saidfourth subcell, said fifth subcell having a sixth band gap smaller thansaid fifth band gap such that said fifth subcell is lattice matched withrespect to said fourth subcell. A second graded interlayer is formedadjacent to said fifth solar subcell; said second graded interlayerhaving a seventh band gap greater than said sixth band gap. A lowersixth solar subcell is formed adjacent to said second graded interlayer,said lower sixth subcell having a eighth band gap smaller than saidsixth band gap such that said sixth subcell is lattice mismatched withrespect to said fifth subcell.

In some embodiments, the base and emitter of the upper first solarsubcell is composed of AlGaInP.

In some embodiments, the band gap of the base of the upper first solarsubcell is equal to or greater than 2.05 eV.

In some embodiments, the emitter of the upper first solar subcell iscomposed of a first region in which the doping is graded from 3×10¹⁸ to1×10¹⁸ free carriers per cubic centimeter, and a second region directlydisposed over the first region in which the doping is constant at 1×10¹⁷free carriers per cubic centimeter.

In some embodiments, the first region of the emitter of the upper firstsolar subcell is directly adjacent to a window layer.

In some embodiments, the emitter of the upper first solar subcell has athickness of 80 nm.

In some embodiments, there is a spacer layer between the emitter and thebase of the upper first solar subcell. In some embodiments, the spacerlayer between the emitter and the base of the upper first solar subcellis composed of unintentionally doped AlGaInP.

In some embodiments, the base of the upper first solar subcell has athickness of less than 400 nm.

In some embodiments, the base of the upper first solar subcell has athickness of 260 nm.

In some embodiments, the emitter section of the upper first solarsubcell has a free carrier density of 3×10¹⁸ to 9×10¹⁸ per cubiccentimeter.

In some embodiments, in particular in connection with a five junctionsolar cell utilizing two metamorphic layers, the band gap of the firstgraded interlayer remains constant at 1.5 eV throughout the thickness ofthe first graded interlayer, and the band gap of the second gradedinterlayer remains constant at 1.5 eV throughout the thickness of thesecond graded interlayer. More specifically, in some embodiments, thepresent disclosure provides a method of manufacturing a solar cell usingan MOCVD process, wherein the first and second graded interlayers arecomposed of (In_(x)Ga_(1-x))_(y)Al_(1-y)As, and are formed in the MOCVDreactor so that they are compositionally graded to lattice match thethird subcell on one side and the fourth subcell on the other side, andthe fourth subcell on one side and the bottom fifth subcell on the otherside, respectively, with the values for x and y computed and thecomposition of the first and second graded interlayers determined sothat as the layers are grown in the MOCVD reactor, the band gap of thefirst graded interlayer remains constant at 1.5 eV throughout thethickness of the first graded interlayer, and the band gap of the secondgraded interlayer remains constant at 1.5 eV throughout the thickness ofthe second graded interlayer.

In some embodiments, in particular in connection with a five junctionsolar cell utilizing two metamorphic layers, the upper subcell iscomposed of an AlGaInP emitter layer and an AlGaInP base layer, thesecond subcell is composed of AlGaAs emitter layer and a AlGaAs baselayer, the third subcell is composed of a GaInP emitter layer and a GaAsbase layer, the fourth subcell is composed of a GaInAs emitter layer anda GaInAs base layer, and the bottom fifth subcell is composed of aGaInAs emitter layer and a GaInAs base layer.

In some embodiments, in particular in connection with a five junctionsolar cell utilizing two metamorphic layers, the lower fifth subcell hasa band gap in the range of approximately 0.85 to 0.95 eV, the fourthsubcell has a band gap in the range of approximately 1.0 to 1.2 eV; thethird subcell has a band gap in the range of approximately 1.3 to 1.5eV, the second subcell has a band gap in the range of approximately 1.65to 1.80 eV and the upper subcell has a band gap in the range of 1.9 to2.2 eV.

In some embodiments, in particular in connection with a six junctionsolar cell utilizing three metamorphic layers, the band gap of the firstgraded interlayer remains constant at 1.5 eV throughout the thickness ofthe first graded interlayer, the band gap of the second gradedinterlayer remains constant at 1.5 eV throughout the thickness of thesecond graded interlayer, and the band gap of the third gradedinterlayer remains constant at 1.1 eV throughout the thickness of thethird graded interlayer.

In some embodiments, in particular in connection with a six junctionsolar cell utilizing three metamorphic layers, the upper subcell iscomposed of an AlGaInP emitter layer and an AlGaInP base layer, thesecond subcell is composed of AlGaAs emitter layer and a AlGaAs baselayer, the third subcell is composed of a GaInP emitter layer and a GaAsbase layer, the fourth subcell is composed of a GaInAs emitter layer anda GaInAs base layer, the fifth subcell is composed of a GaInAs emitterlayer and a GaInAs base layer, and the bottom sixth subcell is composedof a GaInAs emitter layer and a GaInAs base layer.

In some embodiments, in particular in connection with a six junctionsolar cell utilizing three metamorphic layers, the lower sixth subcellhas a band gap in the range of approximately 0.60 to 0.70 eV, the fifthsubcell has a band gap in the range of approximately 0.85 to 0.95 eV thefourth subcell has a band gap in the range of approximately 1.0 to 1.2eV; the third subcell has a band gap in the range of approximately 1.3to 1.5 eV, the second subcell has a band gap in the range ofapproximately 1.65 to 1.80 eV and the upper subcell has a band gap inthe range of 1.9 to 2.2 eV.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph representing the band gap of certain binary materialsand their lattice constants;

FIG. 2A is a cross-sectional view of the solar cell of one embodiment ofa multijunction solar cell after an initial stage of fabricationincluding the deposition of certain semiconductor layers on the growthsubstrate;

FIG. 2B is a cross-sectional view of the solar cell of FIG. 2A after thenext sequence of process steps;

FIG. 2C is a cross-sectional view of the solar cell of FIG. 2B after thenext sequence of process steps;

FIG. 2D is a cross-sectional view of the solar cell of FIG. 2C after thenext sequence of process steps;

FIG. 2E is a cross-sectional view of the solar cell of FIG. 2D after thenext process step;

FIG. 2F is a cross-sectional view of the solar cell of FIG. 2E after thenext process step in which a surrogate substrate is attached;

FIG. 2G is a cross-sectional view of the solar cell of FIG. 2F after thenext process step in which the original substrate is removed;

FIG. 2H is another cross-sectional view of the solar cell of FIG. 2Gwith the surrogate substrate on the bottom of the Figure;

FIG. 3A is a cross-sectional view of the solar cell of an embodiment ofthe present disclosure representing a five junction solar cell utilizingtwo metamorphic layers after an initial stage of fabrication includingthe deposition of certain semiconductor layers on the growth substrate;

FIG. 3B is a cross-sectional view of the solar cell of FIG. 3A after thenext sequence of process steps in which the lower two subcells aregrown;

FIG. 3C is a cross-sectional view of the a solar cell of an embodimentof the present disclosure representing a five junction solar cellutilizing three metamorphic layers after an initial stage of fabricationincluding the deposition of certain semiconductor layers on the growthsubstrate;

FIG. 4 is a cross-sectional view of a solar cell of an embodiment of thepresent disclosure representing a six junction solar cell utilizingthree metamorphic layers after an initial stage of fabrication includingthe deposition of certain semiconductor layers on the growth substrate;

FIG. 5 is a cross-sectional view of a solar cell of another embodimentof the present disclosure representing a five junction solar cellutilizing one metamorphic layer after an initial stage of fabricationincluding the deposition of certain semiconductor layers on the growthsubstrate;

FIG. 6 is a cross-sectional view of a solar cell of another embodimentof the present disclosure representing a six junction solar cellutilizing two metamorphic layers after an initial stage of fabricationincluding the deposition of certain semiconductor layers on the growthsubstrate;

FIG. 7 is a simplified cross-sectional view of the solar cell of eitherFIG. 2H, 3B, 3C, 4, 5, or 6 after the next sequence of process steps inwhich a metallization layer is deposited over the contact layer, and asurrogate substrate attached;

FIG. 8 is a cross-sectional view of the solar cell of FIG. 7 after thenext sequence of process steps in which the growth substrate is removed;

FIG. 9 is a another cross-sectional view of the solar cell of FIG. 7,similar to that of FIG. 8, but here oriented and depicted with thesurrogate substrate at the bottom of the figure;

FIG. 10 is a cross-sectional view of the solar cell of FIG. 9 after thenext sequence of process steps;

FIG. 11 is a cross-sectional view of the solar cell of FIG. 10 after thenext sequence of process steps;

FIG. 12 is a cross-sectional view of the solar cell of FIG. 11 after thenext sequence of process steps;

FIG. 13A is a top plan view of a wafer in one embodiment of the presentdisclosure in which the solar cells are fabricated;

FIG. 13B is a bottom plan view of a wafer in the embodiment of FIG. 13A;

FIG. 14 is a cross-sectional view of the solar cell of FIG. 12 after thenext sequence of process steps;

FIG. 15 is a cross-sectional view of the solar cell of FIG. 14 after thenext sequence of process steps;

FIG. 16 is a top plan view of the wafer of FIG. 15 depicting the surfaceview of the trench etched around the cell in one embodiment of thepresent disclosure;

FIG. 17 is a cross-sectional view of the solar cell of FIG. 15 after thenext sequence of process steps in one embodiment of the presentdisclosure;

FIG. 18 is a cross-sectional view of the solar cell of FIG. 17 after thenext sequence of process steps in one embodiment of the presentdisclosure;

FIG. 19 is a cross-sectional view of the solar cell of FIG. 17 after thenext sequence of process steps in another embodiment of the presentdisclosure;

FIG. 20A is a graph of the doping profile of the emitter and base layersof the top subcell in the solar cell according to the presentdisclosure;

FIG. 20B is a graph of the doping profile of the emitter and base layersof one or more of the middle subcells in the solar cell according to thepresent disclosure;

FIG. 21 is a graph representing the Al, Ga and In mole fractions versusthe Al to In mole fraction in a AlGaInAs material system that isnecessary to achieve a constant 1.5 eV band gap;

FIG. 22 is a diagram representing the relative concentration of Al, In,and Ga in an AlGaInAs material system needed to have a constant band gapwith various designated values (ranging from 0.4 eV to 2.1 eV) asrepresented by curves on the diagram;

FIG. 23 is a graph representing the Ga mole fraction to the Al to Inmole fraction in a AlGaInAs material system that is necessary to achievea constant 1.51 eV band gap;

FIG. 24 is a graph that depicts the current and voltage characteristicsof a test solar cell with the doped emitter structure of FIG. 20Aaccording to the present disclosure, compared to a solar cell with anormally doped emitter structure; and

FIG. 25 is a graph that depicts the external quantum efficiency (EQE) asa function of wavelength of a multijunction solar cell with the dopedemitter structure of FIG. 20A according to the present disclosure,compared with that of a solar cell with a normally doped emitterstructure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

The basic concept of fabricating an inverted metamorphic multijunction(IMM) solar cell is to grow the subcells of the solar cell on asubstrate in a “reverse” sequence. That is, the high band gap subcells(i.e. subcells with band gaps in the range of 1.8 to 2.2 eV), whichwould normally be the “top” subcells facing the solar radiation, aregrown epitaxially on a semiconductor growth substrate, such as forexample GaAs or Ge, and such subcells are therefore lattice matched tosuch substrate. One or more lower band gap middle subcells (i.e. withband gaps in the range of 1.2 to 1.8 eV) can then be grown on the highband gap subcells.

At least one lower subcell is formed over the middle subcell such thatthe at least one lower subcell is substantially lattice mismatched withrespect to the growth substrate and such that the at least one lowersubcell has a third lower band gap (i.e. a band gap in the range of 0.7to 1.2 eV). A surrogate substrate or support structure is then attachedor provided over the “bottom” or substantially lattice mismatched lowersubcell, and the growth semiconductor substrate is subsequently removed.(The growth substrate may then subsequently be re-used for the growth ofa second and subsequent solar cells).

A variety of different features of inverted metamorphic multijunctionsolar cells are disclosed in the related applications noted above. Some,many or all of such features may be included in the structures andprocesses associated with the solar cells of the present disclosure.However, more particularly, the present disclosure is directed to thefabrication of a five or six junction inverted metamorphic solar cellusing either one, two or three different metamorphic layers, all grownon a single growth substrate. In the present disclosure, the resultingconstruction may include four, five, or six subcells, with band gaps inthe range of 1.8 to 2.2 eV (or higher) for the top subcell, and 1.3 to1.8 eV, 0.9 to 1.2 eV for the middle subcells, and 0.6 to 0.8 eV, forthe bottom subcell, respectively.

It should be apparent to one skilled in the art that in addition to theone or two different metamorphic layers discussed in the presentdisclosure, additional types of semiconductor layers within the cell arealso within the scope of the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 is a graph representing the band gap of certain binary materialsand their lattice constants. The band gap and lattice constants ofternary materials are located on the lines drawn between typicalassociated binary materials (such as the ternary material AlGaAs beinglocated between the GaAs and AlAs points on the graph, with the band gapof the ternary material lying between 1.42 eV for GaAs and 2.16 eV forAlAs depending upon the relative amount of the individual constituents).Thus, depending upon the desired band gap, the material constituents ofternary materials can be appropriately selected for growth.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a vapordeposition method, such as Organo Metallic Vapor Phase Epitaxy (OMVPE),Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy(MBE), or other vapor deposition methods for the reverse growth mayenable the layers in the monolithic semiconductor structure forming thecell to be grown with the required thickness, elemental composition,dopant concentration and grading and conductivity type.

The present disclosure is directed to a growth process using a metalorganic chemical vapor deposition (MOCVD) process in a standard,commercially available reactor suitable for high volume production. Moreparticularly, the present disclosure is directed to the materials andfabrication steps that are particularly suitable for producingcommercially viable inverted metamorphic multijunction solar cells usingcommercially available equipment and established high-volume fabricationprocesses, as contrasted with merely academic expositions of laboratoryor experimental results.

In order to provide appropriate background FIG. 2A through 2H depictsthe sequence of steps in forming a four junction solar cell solar cellgenerally as set forth in parent U.S. patent application Ser. No.12/271,192 filed Nov. 14, 2008, herein incorporated by reference.

FIG. 2A depicts the sequential formation of the three subcells A, B andC on a GaAs growth substrate. More particularly, there is shown asubstrate 101, which is preferably gallium arsenide (GaAs), but may alsobe germanium (Ge) or other suitable material. For GaAs, the substrate ispreferably a 15° off-cut substrate, that is to say, its surface isorientated 15° off the (100) plane towards the (111)A plane, as morefully described in U.S. patent application Ser. No. 12/047,944, filedMar. 13, 2008.

In the case of a Ge substrate, a nucleation layer (not shown) isdeposited directly on the substrate 101. On the substrate, or over thenucleation layer (in the case of a Ge substrate), a buffer layer 102 andan etch stop layer 103 are further deposited. In the case of GaAssubstrate, the buffer layer 102 is preferably GaAs. In the case of Gesubstrate, the buffer layer 102 is preferably GaInAs. A contact layer104 of GaAs is then deposited on layer 103, and a window layer 105 ofAlInP is deposited on the contact layer. The subcell A, consisting of ann+ emitter layer 106 and a p-type base layer 107, is then epitaxiallydeposited on the window layer 105. The subcell A is generally latticematched to the growth substrate 101.

It should be noted that the multijunction solar cell structure could beformed by any suitable combination of group III to V elements listed inthe periodic table subject to lattice constant and band gaprequirements, wherein the group III includes boron (B), aluminum (Al),gallium (Ga), indium (In), and thallium (T). The group IV includescarbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group Vincludes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), andbismuth (Bi).

In one embodiment, the emitter layer 106 is composed of GaInP and thebase layer 107 is composed of AlGaInP. In some embodiments, moregenerally, the base-emitter junction may be a heterojunction. In otherembodiments, the base layer may be composed of (Al)GaInP, where thealuminum or Al term in parenthesis in the preceding formula means thatAl is an optional constituent, and in this instance may be used in anamount ranging from 0% to 30%. The doping profile of the emitter andbase layers 106 and 107 according to the present invention will bediscussed in conjunction with FIG. 20.

In some embodiments, the band gap of the base layer 107 is 1.91 eV orgreater.

Subcell A will ultimately become the “top” subcell of the invertedmetamorphic structure after completion of the process steps according tothe present invention to be described hereinafter.

On top of the base layer 107 a back surface field (“BSF”) layer 108preferably p+AlGaInP is deposited and used to reduce recombination loss.

The BSF layer 108 drives minority carriers from the region near thebase/BSF interface surface to minimize the effect of recombination loss.In other words, the BSF layer 18 reduces recombination loss at thebackside of the solar subcell A and thereby reduces the recombination inthe base.

On top of the BSF layer 108 a sequence of heavily doped p-type andn-type layers 109 a and 109 b is deposited that forms a tunnel diode,i.e. an ohmic circuit element that forms an electrical connectionbetween subcell A to subcell B. Layer 109 a is preferably composed ofp++ AlGaAs, and layer 109 b is preferably composed of n++ GaInP.

On top of the tunnel diode layers 109 a window layer 110 is deposited,preferably n+ GaInP. The advantage of utilizing GaInP as the materialconstituent of the window layer 110 is that it has an index ofrefraction that closely matches the adjacent emitter layer 111, as morefully described in U.S. patent application Ser. No. 12/258,190, filedOct. 24, 2008. The window layer 110 used in the subcell B also operatesto reduce the interface recombination loss. It should be apparent to oneskilled in the art, that additional layer(s) may be added or deleted inthe cell structure without departing from the scope of the presentdisclosure.

On top of the window layer 110 the layers of subcell B are deposited:the n-type emitter layer 111 and the p-type base layer 112. These layersare preferably composed of GaInP and GaIn_(0.015)As respectively (for aGe substrate or growth template), or GaInP and GaAs respectively (for aGaAs substrate), although any other suitable materials consistent withlattice constant and band gap requirements may be used as well. Thus,subcell B may be composed of a GaAs, GaInP, GaInAs, GaAsSb, or GaInAsNemitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base region. Thedoping profile of layers 111 and 112 according to the present disclosurewill be discussed in conjunction with FIG. 20B.

In some previously disclosed implementations of an inverted metamorphicsolar cell, the middle cell was a homostructure. In some embodiments ofthe present disclosure, similarly to the structure disclosed in U.S.patent application Ser. No. 12/023,772, the middle subcell becomes aheterostructure with an GaInP emitter and its window is converted fromAlInP to GaInP. This modification eliminated the refractive indexdiscontinuity at the window/emitter interface of the middle subcell, asmore fully described in U.S. patent application Ser. No. 12/258,190,filed Oct. 24, 2008. Moreover, the window layer 110 is preferably dopedthree times that of the emitter 111 to move the Fermi level up closer tothe conduction band and therefore create band bending at thewindow/emitter interface which results in constraining the minoritycarriers to the emitter layer.

In one embodiment of the present disclosure, the middle subcell emitterhas a band gap equal to the top subcell emitter, and the third subcellemitter has a band gap greater than the band gap of the base of themiddle subcell. Therefore, after fabrication of the solar cell, andimplementation and operation, neither the emitters of middle subcell Bnor the third subcell C will be exposed to absorbable radiation.Substantially all of the photons representing absorbable radiation willbe absorbed in the bases of cells B and C, which have narrower band gapsthan the emitters. Therefore, the advantages of using heterojunctionsubcells are: (i) the short wavelength response for both subcells willimprove, and (ii) the bulk of the radiation is more effectively absorbedand collected in the narrower band gap base. The effect will be toincrease the short circuit current J_(sc).

On top of the cell B is deposited a BSF layer 113 which performs thesame function as the BSF layer 109. The p++/n++ tunnel diode layers 114a and 114 b respectively are deposited over the BSF layer 113, similarto the layers 109 a and 109 b, forming an ohmic circuit element toconnect subcell B to subcell C. The layer 114 a may be composed of p++AlGaAs, and layer 114 b may be composed of n++ GaAs or GaInP.

In some embodiments a barrier layer 115, composed of n-type (Al)GaInP,is deposited over the tunnel diode 114 a/114 b, to a thickness of about0.5 micron. Such barrier layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle and top subcells B and C, or in the direction ofgrowth into the bottom subcell A, and is more particularly described incopending U.S. patent application Ser. No. 11/860,183, filed Sep. 24,2007.

A metamorphic layer (or graded interlayer) 116 is deposited over thebarrier layer 115. Layer 116 is preferably a compositionally step-gradedseries of AlGaInAs layers, preferably with monotonically changinglattice constant, so as to achieve a gradual transition in latticeconstant in the semiconductor structure from subcell B to subcell Cwhile minimizing threading dislocations from occurring. In someembodiments, the band gap of layer 116 is constant throughout itsthickness, preferably approximately equal to 1.5 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell B. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.50 eV or other appropriate band gap.

In an alternative embodiment where the solar cell has only two subcells,and the “middle” cell B is the uppermost or top subcell in the finalsolar cell, wherein the “top” subcell B would typically have a band gapof 1.8 to 1.9 eV, then the band gap of the interlayer would remainconstant at 1.9 eV.

In the inverted metamorphic structure described in the Wanlass et al.paper cited above, the metamorphic layer consists of ninecompositionally graded GaInP steps, with each step layer having athickness of 0.25 micron. As a result, each layer of Wanlass et al. hasa different band gap. In one embodiment of the present invention, thelayer 116 is composed of a plurality of layers of AlGaInAs, withmonotonically changing lattice constant, each layer having the same bandgap, approximately 1.5 eV.

The advantage of utilizing a constant band gap material such as AlGaInAsis that arsenide-based semiconductor material is much easier to processfrom a manufacturing standpoint in standard commercial MOCVD reactorsthan materials incorporating phosphorus, while the small amount ofaluminum in the band gap material assures radiation transparency of themetamorphic layers.

Although one embodiment of the present disclosure utilizes a pluralityof layers of AlGaInAs for the metamorphic layer 116 for reasons ofmanufacturability and radiation transparency, other embodiments of thepresent disclosure may utilize different material systems to achieve achange in lattice constant from subcell B to subcell C. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, P, N, Sb based III-Vcompound semiconductors subject to the constraints of having thein-plane lattice parameter greater or equal to that of the second solarcell and less than or equal to that of the third solar cell, and havinga band gap energy greater than that of the second solar cell.

In another embodiment of the present disclosure, an optional secondbarrier layer 117 may be deposited over the AlGaInAs metamorphic layer116. The second barrier layer 117 will typically have a differentcomposition than that of barrier layer 115, and performs essentially thesame function of preventing threading dislocations from propagating. Inone embodiment, barrier layer 117 is n+ type GaInP.

A window layer 118 preferably composed of n+ type GaInP is thendeposited over the barrier layer 117 (or directly over layer 116, in theabsence of a second barrier layer). This window layer operates to reducethe recombination loss in subcell “C”. It should be apparent to oneskilled in the art that additional layers may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

On top of the window layer 118, the layers of cell C are deposited: then+ emitter layer 119, and the p-type base layer 120. These layers arepreferably composed of n+ type GaInAs and p+ type GaInAs respectively,or n+ type GaInP and p type GaInAs for a heterojunction subcell,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers119 and 120 will be discussed in connection with FIG. 20.

A BSF layer 121, preferably composed of AlGaInAs, is then deposited ontop of the cell C, the BSF layer performing the same function as the BSFlayers 108 and 113.

The p++/n++ tunnel diode layers 122 a and 122 b respectively aredeposited over the BSF layer 121, similar to the layers 114 a and 114 b,forming an ohmic circuit element to connect subcell C to subcell D. Thelayer 122 a is composed of p++ AlGaInAs, and layer 122 b is composed ofn++ GaInP.

FIG. 2B depicts a cross-sectional view of the solar cell of FIG. 2Aafter the next sequence of process steps. In some embodiments a barrierlayer 123, preferably composed of n-type GaInP, is deposited over thetunnel diode 122 a/122 b, to a thickness of about 0.5 micron. Suchbarrier layer is intended to prevent threading dislocations frompropagating, either opposite to the direction of growth into the top andmiddle subcells A, B and C, or in the direction of growth into thesubcell D, and is more particularly described in copending U.S. patentapplication Ser. No. 11/860,183, filed Sep. 24, 2007.

A metamorphic layer (or graded interlayer) 124 is deposited over thebarrier layer 123. Layer 124 is preferably a compositionally step-gradedseries of AlGaInAs layers, preferably with monotonically changinglattice constant, so as to achieve a gradual transition in latticeconstant in the semiconductor structure from subcell C to subcell Dwhile minimizing threading dislocations from occurring. In someembodiments the band gap of layer 124 is constant throughout itsthickness, preferably approximately equal to 1.1 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell C. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.1 eV or other appropriate band gap.

A window layer 125 preferably composed of n+ type AlGaInAs is thendeposited over layer 124 (or over a second barrier layer, if there isone, disposed over layer 124,). This window layer operates to reduce therecombination loss in the fourth subcell “D”. It should be apparent toone skilled in the art that additional layers may be added or deleted inthe cell structure without departing from the scope of the presentinvention.

FIG. 2C depicts a cross-sectional view of the solar cell of FIG. 2Bafter the next sequence of process steps. On top of the window layer125, the layers of cell D are deposited: the n+ emitter layer 126, andthe p-type base layer 127. These layers are preferably composed of n+type GaInAs and p type GaInAs respectively, although other suitablematerials consistent with lattice constant and band gap requirements maybe used as well. The doping profile of layers 126 and 127 will bediscussed in connection with FIG. 20.

Turning next to FIG. 2D, a BSF layer 128, preferably composed of p+ typeAlGaInAs, is then deposited on top of the cell D, the BSF layerperforming the same function as the BSF layers 108, 113 and 121.

Finally a high band gap contact layer 129, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 128.

The composition of this contact layer 129 located at the bottom(non-illuminated) side of the lowest band gap photovoltaic cell (i.e.,subcell “D” in the depicted embodiment) in a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (i) the backside ohmic metal contact layerbelow it (on the non-illuminated side) will also act as a mirror layer,and (ii) the contact layer doesn't have to be selectively etched off, toprevent absorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

FIG. 2E is a cross-sectional view of the solar cell of FIG. 2D after thenext process step in which a metal contact layer 130 is deposited overthe p+ semiconductor contact layer 129. The metal is the sequence ofmetal layers Ti/Au/Ag/Au.

Also, the metal contact scheme chosen is one that has a planar interfacewith the semiconductor, after heat treatment to activate the ohmiccontact. This is done so that (1) a dielectric layer separating themetal from the semiconductor doesn't have to be deposited andselectively etched in the metal contact areas; and (2) the contact layeris specularly reflective over the wavelength range of interest.

FIG. 2F is a cross-sectional view of the solar cell of FIG. 2E after thenext process step in which an adhesive layer 131 is deposited over themetal layer 130. The adhesive may be CR 200 (manufactured by BrewerScience, Inc. of Rolla, Mo.).

In the next process step, a surrogate substrate 132, preferablysapphire, is attached. Alternatively, the surrogate substrate may beGaAs, Ge or Si, or other suitable material. The surrogate substrate isabout 40 mils in thickness, and is perforated with holes about 1 mm indiameter, spaced 4 mm apart, to aid in subsequent removal of theadhesive and the substrate. Of course, surrogate substrates with otherthicknesses and perforation configurations may be used as well. As analternative to using an adhesive layer 131, a suitable substrate (e.g.,GaAs) may be eutectically or permanently bonded to the metal layer 130.

FIG. 2G is a cross-sectional view of the solar cell of FIG. 2F after thenext process step in which the original substrate is removed, in oneembodiment, by a sequence of lapping and/or etching steps in which thesubstrate 101, and the buffer layer 103 are removed. The choice of aparticular etchant is growth substrate dependent.

FIG. 2H is a cross-sectional view of the solar cell of FIG. 2G with theorientation with the surrogate substrate 132 being at the bottom of theFigure. Subsequent Figures in this application will assume suchorientation.

Five Junction Solar Cell with Two Metamorphic Layers

FIG. 3A through 3B depicts the sequence of steps in forming amultijunction solar cell in an embodiment according to the presentdisclosure in which a five junction solar cell with two metamorphicbuffer layers is fabricated.

FIG. 3A depicts the initial sequence of steps in forming a multijunctionsolar cell in an embodiment according to the present disclosure in whichthe first three cells of one embodiment of a five junction solar cell isfabricated.

FIG. 3B is a cross-sectional view of the solar cell of FIG. 3A in theembodiment after the next sequence of process steps in which the lowertwo subcells are grown.

FIG. 3C is a cross-sectional view of a solar cell in an embodiment of afive junction solar cell with three metamorphic layers.

Turning first to FIG. 3A, the sequential formation of the three subcellsA, B and C on a GaAs growth substrate is depicted. More particularly,there is shown a substrate 201, which is preferably gallium arsenide(GaAs), but may also be germanium (Ge) or other suitable material. ForGaAs, the substrate is preferably a 15° off-cut substrate, that is tosay, its surface is orientated 15° off the (100) plane towards the(111)A plane, as more fully described in U.S. patent application Ser.No. 12/047,944, filed Mar. 13, 2008.

In the case of a Ge substrate, a nucleation layer (not shown) isdeposited directly on the substrate 201. On the substrate, or over thenucleation layer (in the case of a Ge substrate), a buffer layer 202 andan etch stop layer 203 are further deposited. In the case of GaAssubstrate, the buffer layer 202 is preferably GaAs. In the case of Gesubstrate, the buffer layer 202 is preferably GaInAs. A contact layer204 of GaAs is then deposited on layer 203, and a window layer 205 ofAlInP is deposited on the contact layer. The subcell A, which will bethe upper first solar subcell of the structure, consisting of an n+emitter layer 206 and a p-type base layer 207, is then epitaxiallydeposited on the window layer 205. The subcell A is generally latticematched to the growth substrate 201.

It should be noted that the multijunction solar cell structure could befoamed by any suitable combination of group III to V elements listed inthe periodic table subject to lattice constant and band gaprequirements, wherein the group III includes boron (B), aluminum (Al),gallium (Ga), indium (In), and thallium (T). The group IV includescarbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group Vincludes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), andbismuth (Bi).

In one embodiment, the emitter layer is composed of AlGaInP and the baselayer 207 is composed of AlGaInP, and thus the p/n junction of thissubcell is a homojunction. More particularly, the emitter layer 206 iscomposed of two regions: an n+ type emitter region 206 a directly grownon the window layer 205, and an n type emitter region 206 b directlygrown on the emitter region 206 a. The doping profile of the differentemitter regions 206 a and 206 b, and base layer 207 according to thepresent disclosure will be discussed in conjunction with FIG. 20A.

In some embodiments, a spacer layer 206 c composed of unintentionallydoped AlGaInP is then grown directly on top of the n type emitter region206 b.

The base layer 207 is composed of AlGaInP is grown over the spacer layer206 c.

In some embodiments, the band gap of the base layer 207 is 1.92 eV orgreater.

In some embodiments, the band gap of the base of the upper first solarsubcell is equal to or greater than 2.05 eV.

In some embodiments, the emitter of the upper first solar subcell iscomposed of a first region in which the doping is graded from 3×10¹⁸ to1×10¹⁸ free carriers per cubic centimeter, and a second region directlydisposed over the first region in which the doping is constant at 1×10¹⁷free carriers per cubic centimeter.

In some embodiments, the first region of the emitter of the upper firstsolar subcell is directly adjacent to a window layer.

In some embodiments, the emitter of the upper first solar subcell has athickness of 80 nm.

In some embodiments, the base of the upper first solar subcell has athickness of less than 400 nm.

In some embodiments, the base of the upper first solar subcell has athickness of 260 nm.

In some embodiments, the emitter section of the upper first solarsubcell has a first region in which the doping is graded, and a secondregion directly disposed over the first region in which the doping isconstant.

Subcell A will ultimately become the “top” subcell of the invertedmetamorphic structure after completion of the process steps according tothe present invention to be described hereinafter.

On top of the base layer 207 a back surface field (“BSF”) layer 208preferably p+AlInP is deposited and used to reduce recombination loss.

The BSF layer 208 drives minority carriers from the region near thebase/BSF interface surface to minimize the effect of recombination loss.In other words, the BSF layer 208 reduces recombination loss at thebackside of the solar subcell A and thereby reduces the recombination inthe base.

On top of the BSF layer 208 a sequence of heavily doped p-type andn-type layers 209 a and 209 b is deposited that forms a tunnel diode,i.e. an ohmic circuit element that forms an electrical connectionbetween subcell A to subcell B. Layer 209 a may be composed of p++AlGaAs, and layer 209 b may be composed of n++ GaInP.

On top of the tunnel diode layers 209 a window layer 210 is deposited,which may be n+ AlInP. The advantage of utilizing AlInP as the materialconstituent of the window layer 210 is that it has an index ofrefraction that closely matches the adjacent emitter layer 211, as morefully described in U.S. patent application Ser. No. 12/258,190, filedOct. 24, 2008. The window layer 210 used in the subcell B also operatesto reduce the interface recombination loss. It should be apparent to oneskilled in the art, that additional layer(s) may be added or deleted inthe cell structure without departing from the scope of the presentdisclosure.

On top of the window layer 210 the layers of subcell B are deposited:the n+ type emitter layer 211 and the p-type base layer 212. Theselayers are composed of AlGaAs and AlGaAs respectively (for a GaAssubstrate), although any other suitable materials consistent withlattice constant and band gap requirements may be used as well. Thus,subcell B may be composed of a GaAs, GaInP, GaInAs, GaAsSb, or GaInAsNemitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base region. Thedoping profile of layers 211 and 212 according to the present disclosurewill be discussed in conjunction with FIG. 20B.

In embodiments of the present disclosure, similarly to the structuredisclosed in U.S. patent application Ser. No. 12/023,772, the subcell Bmay be a heterostructure with an GaInP emitter and its window isconverted from AlInP to GaInP. This modification eliminated therefractive index discontinuity at the window/emitter interface of themiddle subcell, as more fully described in U.S. patent application Ser.No. 12/258,190, filed Oct. 24, 2008. Moreover, the window layer 210 ispreferably doped three times that of the emitter 211 to move the Fermilevel up closer to the conduction band and therefore create band bendingat the window/emitter interface which results in constraining theminority carriers to the emitter layer.

On top of the cell B is deposited a BSF layer 213 which performs thesame function as the BSF layer 209. The p++/n++ tunnel diode layers 214a and 214 b respectively are deposited over the BSF layer 213, similarto the layers 209 a and 209 b, forming an ohmic circuit element toconnect subcell B to subcell C. The layer 214 a may be composed ofp++AlGaAs, and layer 214 b may be composed of n++ GaInP

A window layer 215 composed of n+ type GaInP is then deposited over thetunnel diode layers 214 a, 214 b. This window layer operates to reducethe recombination loss in subcell “C”. It should be apparent to oneskilled in the art that additional layers may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

On top of the window layer 215, the layers of subcell C are deposited:the n+ emitter layer 216, and the p-type base layer 217. These layersare composed of n+ type GaInP and p+ type GaAs respectively, althoughother suitable materials consistent with lattice constant and band gaprequirements may be used as well. The doping profile of layers 216 and217 will be discussed in connection with FIG. 27B.

A BSF layer 218, preferably composed of AlGaAs, is then deposited on topof the cell C, the BSF layer performing the same function as the BSFlayers 208 and 213.

The p++/n++ tunnel diode layers 219 a and 219 b respectively aredeposited over the BSF layer 218, similar to the layers 214 a and 214 b,forming an ohmic circuit element to connect subcell C to subcell D. Thelayer 219 a is preferably composed of p AlGaAs, and layer 219 b ispreferably composed of n++ GaInP.

FIG. 3B is a cross-sectional view of the solar cell of FIG. 3A in afirst embodiment of a five junction solar cell after the next sequenceof process steps in which the lower two subcells D and E are grown onthe initial structure of FIG. 3A.

Turning to FIG. 3B, a sequence of layers 220 through 235 are grown ontop of the tunnel diode layers 219 a and 219 b.

In some embodiments a barrier layer 220, composed of n-type (Al)GaInP,is deposited over the tunnel diode 219 a/219 b, to a thickness of about0.5 micron. Such barrier layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells B and C, or in the direction of growthinto the lower subcell D and E, and is more particularly described incopending U.S. patent application Ser. No. 11/860,183, filed Sep. 24,2007.

A first metamorphic layer (or graded interlayer) 221 is deposited overthe barrier layer 220. Layer 221 is preferably a compositionallystep-graded series of AlGaInAs layers, preferably with monotonicallychanging lattice constant, so as to achieve a gradual transition inlattice constant in the semiconductor structure from subcell C tosubcell D while minimizing threading dislocations from occurring. Statedanother way, the layer 221 has a lattice constant on one surfaceadjacent to subcell C which matches that of subcell C, and a latticeconstant on the opposing surface adjacent to subcell D which matchesthat of subcell D, and a gradation in lattice constant throughout itsthickness. In some embodiments, the band gap of layer 221 is constantthroughout its thickness, preferably approximately equal to 1.5 eV, orotherwise consistent with a value slightly greater than the band gap ofthe middle subcell C. One embodiment of the graded interlayer may alsobe expressed as being composed of Al_(y)(Ga_(x)In_(1-x))_(1-y)As, oralternatively written as (In_(x)Ga_(1-x))_(1-y) Al_(y)As, with thepositive values for x and y selected such that the band gap of theinterlayer remains constant at approximately 1.50 eV or otherappropriate band gap.

In the inverted metamorphic structure described in the Wanlass et al.paper cited above, the metamorphic layer consists of ninecompositionally graded GaInP steps, with each step layer having athickness of 0.25 micron. As a result, each layer of Wanlass et al. hasa different band gap. In one embodiment of the present disclosure, thelayer 221 is composed of a plurality of layers of AlGaInAs, withmonotonically changing lattice constant, each layer having the same bandgap, approximately 1.5 eV. More particularly, the first gradedinterlayer 221 is composed of Al_(y)(Ga_(x) In_(1-x))_(1-y)As with thevalues for x and y between 0 and 1 selected such that the band gap ofeach sublayer in the interlayer remains constant throughout itsthickness.

In some embodiments, the band gap of the first graded interlayer remainsconstant at 1.5 eV throughout the thickness of the first gradedinterlayer.

Since the present disclosure (and the related applications noted above)are directed to high volume manufacturing processes using metalorganicvapor phase epitaxy (MOVPE) reactors to form the solar cell epitaxiallayers, a short discussion of some of the considerations associated withsuch processes and methods associated with the formation of the gradedinterlayer(s) are in order here.

First, it should be noted that the advantage of utilizing an interlayermaterial such as AlGaInAs is that arsenide-based semiconductor materialis much easier to process from a manufacturing standpoint using presentstate-of-the-art high volume manufacturing metalorganic vapor phaseepitaxy (MOVPE) reactors than either the AlGaInAsP, or GaInP compounds,or in general any material including phosphorus. Simply stated, the useof a III-V arsenide compound is much more desirable than a III-Vphosphide compound from the perspectives of cost, ease of growth,reactor maintenance, waste handling and personal safety.

The cost advantage of the use of the AlGaInAs quaternary gradingmaterial relative to a GaInP ternary grading material, as an example, isa consequence of several factors. First, the use of a GaInP gradingapproach requires indium mole fractions of the order of 60% (i.e., therequired material is Ga_(0.4)In_(0.6)P) whereas the use of the AlGaInAsquaternary requires only 15% indium (i.e., the required material isAl_(y)(Ga_(0.85)In_(0.15))_(1-y)As). In addition to the difference inthe material itself, there is a further difference in the amount ofprecursor gases (trimethylgallium, trimethylindium, and arsine) thatmust be input to the reactor in order to achieve the desiredcomposition. In particular, the ratio of the amount of precursor gasesinto the reactor to provide Group V elements, to the amount of precursorgases to provide Group III elements (such ratio being referred to as the“input VIII ratio”) is typically five to ten times greater to produce aphosphide compound compared to producing an arsenide compound. As aillustrative quantification of the cost of producing a phosphidecompound in a commercial operational MOPVE reactor process compared tothe cost of producing an arsenide compound, Table 1 below presents thetypical pro-forma costs of each element of the AlGaInAs and GaInPcompounds for producing a graded interlayer of the type described in thepresent disclosure expressed on a per mole basis. Of course, like manycommodities, the price of chemical compounds fluctuate from time to timeand vary in different geographic locations and countries and fromsupplier to supplier. The prices used in Table 1 are representative forpurchases in commercial quantities in the United States at the time ofthe present disclosure. The cost calculations make the assumption(typical for epitaxial processes using current commercial MOVPEreactors) that the input VIII ratios are 20 and 120 for the arsenide andphosphide compounds respectively. Such a choice of value of the ratio ismerely illustrative for a typical process, and some processes may useeven higher ratios for producing a graded interlayer of the typedescribed in the present disclosure. The practical consequence of theinlet VIII ratio is that one will use 20 moles of As to one (1) mole ofAlGaIn in the formation of the Applicant's quaternary material AlGaInAs,or 120 moles of P to 1 mole of Gain in the formation of the interlayerusing the ternary material GaInP. These assumptions along with the molarcost of each of the constituent elements indicate that the cost offabrication of the AlGaInAs based grading interlayer will beapproximately 25% of the cost of fabrication of a similar phosphidebased grading interlayer. Thus, there is an important economic incentivefrom a commercial and manufacturing perspective to utilize an arsenidecompound as opposed to a phosphide compound for the grading interlayer.

TABLE 1 Cost estimate of one mole of each of the AlGaInAs and GAInPgrading layers Cost Molecular Mole Cost Molecular Mole Element MW (gms)$/gm Cost/mole ($) MF AlGaIn of Al.17Ga.68In.15 MF GaInP of Ga.4In.6 Al27 10.2 275.4 0.17 46.818 0 0 Ga 70 2.68 187.6 0.68 127.568 0.4 75.04 In115 28.05 3225.75 0.15 483.8625 0.6 1935.45 Approx OM 658.2485 2010.49Cost/mole = Cost/mole V/III Cost/mole of ($) ratio Cost/mole of Arsenicphosphorus AsH3 $7.56 20   $151.20 $151.20 PH3 $9.49 120 $1,138.80$1,138.54 Approx $809.45 $3,149.03 cost/molecular mole =

The “ease of growth” of an arsenide compound as opposed to a phosphidecompound for the grading interlayer in a high volume manufacturingenvironment is another important consideration and is closely related toissues of reactor maintenance, waste handling and personal safety. Moreparticularly, in a high volume manufacturing environment the abatementdifferences between arsenide and phosphide based processes affect bothcost and safety. The abatement of phosphorus is more time consuming, andhazardous than that required for arsenic. Each of these compounds buildsup over time in the downstream gas flow portions of the MOVPE growthreactor. As such, periodic reactor maintenance for removal of thesedeposited materials is necessary to prevent adverse affects on thereactor flow dynamics, and thus the repeatability and uniformity of theepitaxial structures grown in the reactor. The difference in handling ofthese waste materials is significant. Arsenic as a compound is stable inair, non-flammable, and only represents a mild irritant upon skincontact. Phosphorus however, must be handled with considerably morecare. Phosphorus is very flammable and produces toxic fumes upon burningand it is only moderately stable in air. Essentially the differences aremanifest by the need for special handling and containment materials andprocedures when handling phosphorus to prevent either combustion ortoxic exposure to this material whereas using common personal protectionequipment such as gloves, and a particle respirator easily accommodatesthe handling of arsenic.

Another consideration related to “ease of growth” that should be notedin connection with the advantages of a AlGaInAs based grading interlayerprocess compared to a AlGaInAsP compound derives from a frequentlyencountered issue when using an AlGaInAsP compound: the miscibility gap.A miscibility gap will occur if the enthalpy of mixing exceeds theentropy of mixing of two binary compounds AC and BC, where A, B and Care different elements. It is an established fact that the enthalpies ofmixing of all ternary crystalline alloys of the form A_(x)B_(1-x)C,based upon the binary semiconductor forms AC and BC are positive leadingto miscibility gaps in these compounds. See, for example, the discussionin reference [1] noted below. In this example, the letters A and Bdesignate group III elements and letter C designates a group V element.As such, mixing of the binary compounds is said to occur on the groupIII sublattice. However, because OMVPE growth takes place undernon-equilibrium conditions, the miscibility gap is not really apractical problem for accessing the entire ternary semiconductor phasespace. For the case of quaternary compounds of the formA_(x)B_(1-x)C_(y)D_(1-y) where mixing of the binary alloys, AC, AD, BC,and BD occurs on both the group III and group V sublattices, theimmiscibility problem is accentuated. Specifically for the GaP, InP,GaAs, InAs system, the region of immiscibility is quite large at growthtemperatures appropriate for the OMVPE technique. See, for example, thediscussion in reference [2] noted below. The resulting miscibility gapwill prevent one from producing the requisite AlGaInAsP compounds neededfor optical transparent grading of the IMM solar cell.

References:

-   [1] V. A. Elyukhin, E. L. Portnoi, E. A. Avrutin, and J. H.    Marsh, J. Crystal Growth 173 (1997) pp 69-72.-   [2] G. B. Stringfellow, Organometallic Vapor-Phase Epitaxy (Academic    Press, New York 1989).

The fabrication of a step graded (or continuous graded) interlayer in anMOCVD process can be more explicitly described in a sequence ofconceptual and operational steps which we describe here for pedagogicalclarity. First, the appropriate band gap for the interlayer must beselected. In one of the disclosed embodiments, the desired constant bandgap is 1.5 eV. Second, the most appropriate material system (i.e., thespecific semiconductor elements to form a compound semiconductor alloy)must be identified. In the disclosed embodiment, these elements are Al,Ga, In, and As. Third, a computation must be made, for example using acomputer program, to identify the class of compounds ofAl_(y)(Ga_(x)In_(1-x))_(1-y)As for specific x and y that have a band gapof 1.5 eV. An example of such a computer program output that provides avery rough indication of these compounds is illustrated in FIG. 22.Fourth, based upon the lattice constants of the epitaxial layersadjoining the graded interlayer, a specification of the required latticeconstants for the top surface of the interlayer (to match the adjacentsemiconductor layer), and the bottom surface of the interlayer (to matchthe adjacent semiconductor layer) must be made. Fifth, based on therequired lattice constants, the compounds of Al_(y)(Ga_(x)In_(1-x))_(1-y)As for specific x and y that have a band gap of 1.5 eVmay be identified. Again, a computation must be made, and as an example,the data may be displayed in a graph such as FIG. 21 representing theAl, Ga and In mole fractions versus the Al to In mole fraction in aAlGaInAs material system that is necessary to achieve a constant 1.5 eVband gap. Assuming there is a small number (e.g. typically in the rangeof seven, eight, nine, ten, etc.) of steps or grades between the topsurface and the bottom surface, and that the lattice constant differencebetween each step is made equal, the bold markings in FIG. 21 representselected lattice constants for each successive sublayer in theinterlayer, and the corresponding mole fraction of Al, Ga and In neededto achieve that lattice constant in that respective sublayer may bereadily obtained by reference to the axes of the graph. Thus, based onan analysis of the data in FIG. 21, the reactor may be programmed tointroduce the appropriate quantities of precursor gases (as determinedby flow rates at certain timed intervals) into the reactor so as toachieve a desired specific Al_(y)(Ga_(x)In_(1-x))_(1-y)As composition inthat sublayer so that each successive sublayer maintains the constantband gap of 1.5 eV and a monotonically increasing lattice constant. Theexecution of this sequence of steps, with calculated and determinateprecursor gas composition, flow rate, temperature, and reactor time toachieve the growth of a Al_(y)(Ga_(x)In_(1-x))_(1-y)As composition ofthe interlayer with the desired properties (lattice constant change overthickness, constant band gap over the entire thickness), in arepeatable, manufacturable process, is not to be minimalized ortrivialized.

Although one embodiment of the present disclosure utilizes a pluralityof layers of AlGaInAs for the metamorphic layer 221 for reasons ofmanufacturability and radiation transparency, other embodiments of thepresent disclosure may utilize different material systems to achieve achange in lattice constant from subcell C to subcell D. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, N, Sb based III-V compoundsemiconductors subject to the constraints of having the in-plane latticeparameter greater or equal to that of the third solar cell and less thanor equal to that of the fourth solar cell, and having a band gap energygreater than that of the third solar cell.

In one embodiment of the present disclosure, an optional second barrierlayer 222 may be deposited over the AlGaInAs metamorphic layer 221. Thesecond barrier layer 222 will typically have a different compositionthan that of barrier layer 220, and performs essentially the samefunction of preventing threading dislocations from propagating. In oneembodiment, barrier layer 222 is n+ type GaInP.

A window layer 223 preferably composed of n+ type GaInP is thendeposited over the second barrier layer, if there is one, disposed overlayer 221. This window layer operates to reduce the recombination lossin the fourth subcell “D”. It should be apparent to one skilled in theart that additional layers may be added or deleted in the cell structurewithout departing from the scope of the present invention.

On top of the window layer 223, the layers of cell D are deposited: then+ emitter layer 224, and the p-type base layer 225. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers224 and 225 will be discussed in connection with FIG. 20B.

A BSF layer 226, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell D, the BSF layer performing the samefunction as the BSF layers 208, 213 and 218.

The p++/n++ tunnel diode layers 227 a and 227 b respectively aredeposited over the BSF layer 226, similar to the layers 214 a/214 b and219 a/219 b, forming an ohmic circuit element to connect subcell D tosubcell E. The layer 227 a is preferably composed of p++ AlGaInAs, andlayer 227 b is preferably composed of n++ GaInP.

In some embodiments a barrier layer 228, preferably composed of n-typeGaInP, is deposited over the tunnel diode 227 a/227 b, to a thickness ofabout 0.5 micron. Such bather layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells B, C and D, or in the direction ofgrowth into the subcell E, and is more particularly described incopending U.S. patent application Ser. No. 11/860,183, filed Sep. 24,2007.

A second metamorphic layer (or graded interlayer) 229 is deposited overthe bather layer 228. Layer 229 is preferably a compositionallystep-graded series of AlGaInAs layers, preferably with monotonicallychanging lattice constant, so as to achieve a gradual transition inlattice constant in the semiconductor structure from subcell D tosubcell E while minimizing threading dislocations from occurring. Insome embodiments the band gap of layer 229 is constant throughout itsthickness, preferably approximately equal to 1.1 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell D. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.1 eV or other appropriate band gap.

In one embodiment of the present disclosure, an optional second barrierlayer 230 may be deposited over the AlGaInAs metamorphic layer 229. Thesecond barrier layer 230 performs essentially the same function as thefirst barrier layer 228 of preventing threading dislocations frompropagating. In one embodiment, barrier layer 230 has not the samecomposition than that of barrier layer 228, i.e. n+ type GaInP

A window layer 231 preferably composed of n+ type GaInP is thendeposited over the barrier layer 230. This window layer operates toreduce the recombination loss in the fifth subcell “E”. It should beapparent to one skilled in the art that additional layers may be addedor deleted in the cell structure without departing from the scope of thepresent invention.

On top of the window layer 231, the layers of cell E are deposited: then+ emitter layer 232, and the p-type base layer 233. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers232 and 233 will be discussed in connection with FIG. 20B.

A BSF layer 234, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell E, the BSF layer performing the samefunction as the BSF layers 208, 213, 218, and 226.

Finally a high band gap contact layer 235, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 234.

The composition of this contact layer 235 located at the bottom(non-illuminated) side of the lowest band gap photovoltaic cell (i.e.,subcell “E” in the depicted embodiment) in a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (i) the backside ohmic metal contact layerbelow it (on the non-illuminated side) will also act as a mirror layer,and (ii) the contact layer doesn't have to be selectively etched off, toprevent absorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

The subsequent remaining steps in the fabrication of the multijunctionsolar cell according to the illustrated embodiment, including thedeposition of metallization over the contact layer, and the attachmentof a surrogate substrate, will be discussed and depicted at a laterpoint in connection with FIG. 7 and subsequent Figures.

Five Junction Solar Cell with Three Metamorphic Layers

FIG. 3C is a cross-sectional view of the solar cell of FIG. 3A in anembodiment of a five junction solar cell with three metamorphic layers.The layers 201 through 214 b of this embodiment are substantiallyidentical to those discussed in connection with the embodiment of FIG.3A, and therefore in the interest of brevity of this disclosure, thedescription of such layers will not be repeated here.

As depicted in FIG. 3C, the embodiment of a five junction solar cellwith three metamorphic layers, a sequence of layers 250 through 273 aregrown on top of the tunnel diode layers 214 a and 214 b of the structureof FIG. 3A.

In some embodiments a barrier layer 250, composed of n-type (Al)GaInP,is deposited over the tunnel diode 214 a/214 b, to a thickness of about1.0 micron. Such barrier layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the subcells A and B, or in the direction of growth into themiddle subcells C and D, and is more particularly described in copendingU.S. patent application Ser. No. 11/860,183, filed Sep. 24, 2007.

A first metamorphic layer (or graded interlayer) 251 is deposited overthe barrier layer 250. Layer 251 is preferably a compositionallystep-graded series of AlGaInAs layers, preferably with monotonicallychanging lattice constant, so as to achieve a gradual transition inlattice constant in the semiconductor structure from subcell B tosubcell C while minimizing threading dislocations from occurring. Statedanother way, the layer 251 has a lattice constant on one surfaceadjacent to subcell B which matches that of subcell B, and a latticeconstant on the opposing surface adjacent to subcell C which matchesthat of subcell C, and a gradation in lattice constant throughout itsthickness. In some embodiments, the band gap of layer 251 is constantthroughout its thickness, preferably approximately equal to 1.5 eV, orotherwise consistent with a value slightly greater than the band gap ofthe middle subcell B. One embodiment of the graded interlayer may alsobe expressed as being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, withx and y selected such that the band gap of the interlayer remainsconstant at approximately 1.50 eV or other appropriate band gap.

In the inverted metamorphic structure described in the Wanlass et al.paper cited above, the metamorphic layer consists of ninecompositionally graded GaInP steps, with each step layer having athickness of 0.25 micron. As a result, each layer of Wanlass et al. hasa different band gap. In one embodiment of the present disclosure, thelayer 251 is composed of a plurality of layers of AlGaInAs, withmonotonically changing lattice constant, each layer having the same bandgap, approximately 1.5 eV.

The advantage of utilizing a constant band gap material such as AlGaInAsis that arsenide-based semiconductor material is much easier to processfrom a manufacturing standpoint in standard commercial MOCVD reactorsthan materials incorporating phosphorus, while the small amount ofaluminum in the band gap material assures radiation transparency of themetamorphic layers.

Although one embodiment of the present disclosure utilizes a pluralityof layers of AlGaInAs for the metamorphic layer 251 for reasons ofmanufacturability and radiation transparency, other embodiments of thepresent disclosure may utilize different material systems to achieve achange in lattice constant from subcell B to subcell C. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, N, Sb based III-V compoundsemiconductors subject to the constraints of having the in-plane latticeparameter greater or equal to that of the second solar subcell and lessthan or equal to that of the third solar subcellcell, and having a bandgap energy greater than that of the third solar cell.

In one embodiment of the present disclosure, an optional second barrierlayer 252 may be deposited over the AlGaInAs metamorphic layer 251. Thesecond barrier layer 252 will typically have a different compositionthan that of barrier layer 250, and performs essentially the samefunction of preventing threading dislocations from propagating. In oneembodiment, barrier layer 252 is n+ type GaInP

A window layer 253 composed of n+ type GaInP is then deposited over thebarrier layer 253. This window layer operates to reduce therecombination loss in the third subcell “C”. It should be apparent toone skilled in the art that additional layers may be added or deleted inthe cell structure without departing from the scope of the presentinvention.

On top of the window layer 253, the layers of cell C are deposited: then+ emitter layer 254, and the p-type base layer 255. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers254 and 255 will be discussed in connection with FIG. 20B.

A BSF layer 256, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell C, the BSF layer performing the samefunction as the BSF layers 208, and 213.

The p++/n++ tunnel diode layers 257 a and 257 b respectively aredeposited over the BSF layer 256, similar to the layers 209 a/209 b and214 a/214 b, forming an ohmic circuit element to connect subcell C tosubcell D. The layer 257 a is preferably composed of p++AlGaInAs, andlayer 257 b is preferably composed of n++ AlGaInAs

In some embodiments a barrier layer 258, may be composed of n-typeGaInP, is deposited over the tunnel diode 257 a/257 b, to a thickness ofabout 0.5 micron. Such barrier layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells B and C, or in the direction of growthinto the subcell D, and is more particularly described in copending U.S.patent application Ser. No. 11/860,183, filed Sep. 24, 2007.

A second metamorphic layer (or graded interlayer) 259 is deposited overthe barrier layer 258. Layer 259 may be a compositionally step-gradedseries of AlGaInAs layers, preferably with monotonically changinglattice constant, so as to achieve a gradual transition in latticeconstant in the semiconductor structure from subcell C to subcell Dwhile minimizing threading dislocations from occurring. In someembodiments the band gap of layer 259 is constant throughout itsthickness, preferably approximately equal to 1.1 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell C. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.1 eV or other appropriate band gap.

In one embodiment of the present disclosure, an optional second batherlayer 260 may be deposited over the AlGaInAs metamorphic layer 259. Thesecond barrier layer will typically have a different composition thanthat of bather layer 258, and performs essentially the same function ofpreventing threading dislocations from propagating.

A window layer 261 composed of n+ type GaInP is then deposited over thebather layer 260. This window layer operates to reduce the recombinationloss in the subcell “D”. It should be apparent to one skilled in the artthat additional layers may be added or deleted in the cell structurewithout departing from the scope of the present invention.

On top of the window layer 260, the layers of cell D are deposited: then+ emitter layer 262, and the p-type base layer 263. These layers areillustrated as being composed of n+ type GaInAs and p type GaInAsrespectively, although other suitable materials consistent with latticeconstant and band gap requirements may be used as well. The dopingprofile of layers 260 and 261 will be discussed in connection with FIG.20B.

A BSF layer 264, may be composed of p+ type AlGaInAs, is then depositedon top of the cell D, the BSF layer performing the same function as theBSF layers 208, 213, and 256.

The p++/n++ tunnel diode layers 265 a and 265 b respectively aredeposited over the BSF layer 264, similar to the layers 209 a/209 b, 214a/214 b, and 257 a/257 b, forming an ohmic circuit element to connectsubcell D to subcell E. The layer 265 a may be composed of p++AlGaInAs,and layer 265 b may be composed of n++ AlGaInAs.

In some embodiments a barrier layer 266, may be composed of n-typeGaInP, is deposited over the tunnel diode 265 a/265 b, to a thickness ofabout 0.5 micron. Such barrier layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells B, C and D, or in the direction ofgrowth into the subcell E, and is more particularly described incopending U.S. patent application Ser. No. 11/860,183, filed Sep. 24,2007.

A third metamorphic layer (or graded interlayer) 267 is deposited overthe barrier layer 266. Layer 267 may be a compositionally step-gradedseries of AlGaInAs layers, preferably with monotonically changinglattice constant, so as to achieve a gradual transition in latticeconstant in the semiconductor structure from subcell D to subcell Ewhile minimizing threading dislocations from occurring. In someembodiments the band gap of layer 267 is constant throughout itsthickness, preferably approximately equal to 1.1 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell D. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y)Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.1 eV or other appropriate band gap.

In one embodiment of the present disclosure, an optional second barrierlayer 268 may be deposited over the AlGaInAs metamorphic layer 267. Thesecond barrier layer is a different alloy than that of bather layer 266,and performs essentially the same function of preventing threadingdislocations from propagating.

A window layer 269 preferably composed of n+ type AlGaInAs is thendeposited over the second bather layer 268, if there is one disposedover layer 267, or directly over third metamorphic layer 267. Thiswindow layer operates to reduce the recombination loss in the fifthsubcell “E”. It should be apparent to one skilled in the art thatadditional layers may be added or deleted in the cell structure withoutdeparting from the scope of the present invention.

On top of the window layer 269, the layers of cell E are deposited: then+ emitter layer 270, and the p-type base layer 271. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers270 and 271 will be discussed in connection with FIG. 20B.

A BSF layer 272, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell E, the BSF layer performing the samefunction as the BSF layers 208, 213, 256, and 264.

Finally a high band gap contact layer 273, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 272.

The composition of this contact layer 273 located at the bottom(non-illuminated) side of the lowest band gap photovoltaic cell (i.e.,subcell “E” in the depicted embodiment) in a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (i) the backside ohmic metal contact layerbelow it (on the non-illuminated side) will also act as a mirror layer,and (ii) the contact layer doesn't have to be selectively etched off, toprevent absorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

The subsequent remaining steps in the fabrication of the multijunctionsolar cell according to the illustrated embodiment, including thedeposition of metallization over the contact layer, and the attachmentof a surrogate substrate, will be discussed and depicted at a laterpoint in connection with FIG. 7 and subsequent Figures.

Six Junction Solar Cell with Three Metamorphic Layers

FIG. 4 depicts a multijunction solar cell in an embodiment according tothe present disclosure in which a six junction solar cell with threemetamorphic buffer layers is fabricated.

In particular, FIG. 4 depicts the sequential formation of the sixsubcells A, B, C, D, E and F on a GaAs growth substrate. The sequence oflayers 302 through 314 b that are grown on the growth substrate aresimilar to layers 102 to 122 b discussed in connection with FIG. 3A, butthe description of such layers with new reference numbers will berepeated here for clarity of the presentation.

More particularly, there is shown a substrate 301, which is preferablygallium arsenide (GaAs), but may also be germanium (Ge) or othersuitable material. For GaAs, the substrate is preferably a 15° off-cutsubstrate, that is to say, its surface is orientated 15° off the (100)plane towards the (111)A plane, as more fully described in U.S. patentapplication Ser. No. 12/047,944, filed Mar. 13, 2008.

In the case of a Ge substrate, a nucleation layer (not shown) isdeposited directly on the substrate 301. On the substrate, or over thenucleation layer (in the case of a Ge substrate), a buffer layer 302 andan etch stop layer 303 are further deposited. In the case of GaAssubstrate, the buffer layer 302 is preferably GaAs. In the case of Gesubstrate, the buffer layer 302 is preferably GaInAs. A contact layer304 of GaAs is then deposited on layer 303, and a window layer 305 ofAlInP is deposited on the contact layer. The subcell A, which will bethe upper first solar subcell of the structure, consisting of an n+emitter layer 306 a and 306 b and a p-type base layer 307, is thenepitaxially deposited on the window layer 305. The subcell A isgenerally lattice matched to the growth substrate 301.

It should be noted that the multijunction solar cell structure could beformed by any suitable combination of group III to V elements listed inthe periodic table subject to lattice constant and band gaprequirements, wherein the group III includes boron (B), aluminum (Al),gallium (Ga), indium (In), and thallium (T). The group IV includescarbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group Vincludes nitrogen (N), phosphorus (P), arsenic

(As), antimony (Sb), and bismuth (Bi).

In one embodiment, the emitter layer is composed of AlGaInP and the baselayer 307 is composed of AlGaInP, and thus the p/n junction of thissubcell is a homojunction. More particularly, the emitter layer iscomposed of two regions: an n+ type emitter region 306 a directly grownon the window layer 305, and an n type emitter region 306 b directlygrown on the emitter region 306 a. The doping profile of the differentemitter regions 306 a and 306 b, and base layer 307 according to thepresent disclosure will be discussed in conjunction with FIG. 20A.

In some embodiments, a spacer layer 306 c composed of unintentionallydoped AlGaInP is then grown directly on top of the n type emitter region306 b.

The base layer 307 is composed of AlGaInP is grown over the spacer layer306 c.

In some embodiments, the band gap of the base layer 307 is 1.92 eV orgreater.

In some embodiments, the band gap of the base of the upper first solarsubcell is equal to or greater than 2.05 eV.

In some embodiments, the emitter of the upper first solar subcell iscomposed of a first region in which the doping is graded from 3×10¹⁸ to1×10¹⁸ free carriers per cubic centimeter, and a second region directlydisposed over the first region in which the doping is constant at 1×10¹⁷free carriers per cubic centimeter.

In some embodiments, the first region of the emitter of the upper firstsolar subcell is directly adjacent to a window layer.

In some embodiments, the emitter of the upper first solar subcell has athickness of 80 nm.

In some embodiments, the base of the upper first solar subcell has athickness of less than 400 nm.

In some embodiments, the base of the upper first solar subcell has athickness of 260 nm.

In some embodiments, the emitter section of the upper first solarsubcell has a first region in which the doping is graded, and a secondregion directly disposed over the first region in which the doping isconstant.

Subcell A will ultimately become the “top” subcell of the invertedmetamorphic structure after completion of the process steps according tothe present invention to be described hereinafter.

On top of the base layer 307 a back surface field (“BSF”) layer 308preferably p+AlInP is deposited and used to reduce recombination loss.

The BSF layer 308 drives minority carriers from the region near thebase/BSF interface surface to minimize the effect of recombination loss.In other words, the BSF layer 308 reduces recombination loss at thebackside of the solar subcell A and thereby reduces the recombination inthe base.

On top of the BSF layer 308 a sequence of heavily doped p-type andn-type layers 309 a and 309 b is deposited that forms a tunnel diode,i.e. an ohmic circuit element that forms an electrical connectionbetween subcell A to subcell B. Layer 309 a may be composed of p++AlGaAs, and layer 309 b may be composed of n++GaInP.

On top of the tunnel diode layers 309 a window layer 310 is deposited,which may be n+ AlInP. The window layer 310 used in the subcell B alsooperates to reduce the interface recombination loss. It should beapparent to one skilled in the art, that additional layer(s) may beadded or deleted in the cell structure without departing from the scopeof the present disclosure.

On top of the window layer 310 the layers of subcell B are deposited:the n+ type emitter layer 311 and the p-type base layer 312. Theselayers are preferably composed of AlGaAs and AlGaAs respectively,although any other suitable materials consistent with lattice constantand band gap requirements may be used as well. Thus, subcell B may becomposed of a GaAs, GaInP, GaInAs, GaAsSb, or GaInAsN emitter region anda GaAs, GaInAs, GaAsSb, or GaInAsN base region. The doping profile oflayers 311 and 312 according to the present disclosure will be discussedin conjunction with FIG. 20B.

On top of the cell B is deposited a BSF layer 313 which performs thesame function as the BSF layer 308. The p++/n++ tunnel diode layers 314a and 314 b respectively are deposited over the BSF layer 308, similarto the layers 309 a and 309 b, forming an ohmic circuit element toconnect subcell B to subcell C. The layer 314 a may be composed of p++AlGaAs, and layer 314 b may be composed of n++ GaInP.

A window layer 315 preferably composed of n+ type GaInP is thendeposited over the tunnel diode layers 314 a/314 b. This window layeroperates to reduce the recombination loss in subcell “C”. It should beapparent to one skilled in the art that additional layers may be addedor deleted in the cell structure without departing from the scope of thepresent disclosure.

On top of the window layer 315, the layers of subcell C are deposited:the n+ emitter layer 316, and the p-type base layer 317. These layersare preferably composed of n+ type GaInP and p type GaAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers316 and 317 will be discussed in connection with FIG. 20B.

A BSF layer 318, preferably composed of AlGaAs, is then deposited on topof the cell C, the BSF layer performing the same function as the BSFlayers 308 and 313.

The p++/n++ tunnel diode layers 319 a and 319 b respectively aredeposited over the BSF layer 318, similar to the layers 314 a and 314 b,forming an ohmic circuit element to connect subcell C to subcell D. Thelayer 319 a is preferably composed of p++ AlGaAs, and layer 319 b may becomposed of n GaAs.

In some embodiments a barrier layer 320, preferably composed of n-typeGaInP, is deposited over the tunnel diode 319 a/319 b, to a thickness ofabout 0.5 micron. Such barrier layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the top and middle subcells A, B and C, or in the directionof growth into the subcell D, and is more particularly described incopending U.S. patent application Ser. No. 11/860,183, filed Sep. 24,2007.

A first metamorphic layer (or graded interlayer) 321 is deposited overthe barrier layer 320. Layer 321 is preferably a compositionallystep-graded series of AlGaInAs layers, preferably with monotonicallychanging lattice constant, so as to achieve a gradual transition inlattice constant in the semiconductor structure from subcell C tosubcell D while minimizing threading dislocations from occurring. Insome embodiments the band gap of layer 321 is constant throughout itsthickness, preferably approximately equal to 1.5 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell C. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.5 eV or other appropriate band gap.

In one embodiment of the present disclosure, an optional second barrierlayer 322 may be deposited over the AlGaInAs metamorphic layer 321. Thesecond barrier layer 322 will typically have a different compositionthan that of barrier layer 320, and performs essentially the samefunction of preventing threading dislocations from propagating. In oneembodiment, barrier layer 322 is n+ type GaInP

A window layer 323 preferably composed of n+ type Gala′ is thendeposited over the second bather layer, if there is one, disposed overlayer 322. This window layer operates to reduce the recombination lossin the fourth subcell “D”. It should be apparent to one skilled in theart that additional layers may be added or deleted in the cell structurewithout departing from the scope of the present invention.

On top of the window layer 323, the layers of cell D are deposited: then+ emitter layer 324, and the p-type base layer 325. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers324 and 325 will be discussed in connection with FIG. 20B.

A BSF layer 326, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell D, the BSF layer performing the samefunction as the BSF layers 308, 313, and 318.

The p++/n++ tunnel diode layers 327 a and 327 b respectively aredeposited over the BSF layer 326, similar to the layers 309 a/309 b, and319 a/319 b, forming an ohmic circuit element to connect subcell D tosubcell E. The layer 327 a is preferably composed of p++ AlGaInAs, andlayer 327 b is preferably composed of n++ GaInP.

In some embodiments a barrier layer 328, preferably composed of n-typeGaInP, is deposited over the tunnel diode 327 a/327 b, to a thickness ofabout 0.5 micron. Such barrier layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells B, C and D, or in the direction ofgrowth into the subcell E, and is more particularly described incopending U.S. patent application Ser. No. 11/860,183, filed Sep. 24,2007.

A second metamorphic layer (or graded interlayer) 329 is deposited overthe bather layer 328. Layer 329 is preferably a compositionallystep-graded series of AlGaInAs layers, preferably with monotonicallychanging lattice constant, so as to achieve a gradual transition inlattice constant in the semiconductor structure from subcell D tosubcell E while minimizing threading dislocations from occurring. Insome embodiments the band gap of layer 329 is constant throughout itsthickness, preferably approximately equal to 1.5 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell D. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.5 eV or other appropriate band gap.

In one embodiment of the present disclosure, an optional second barrierlayer 330 may be deposited over the AlGaInAs metamorphic layer 329. Thesecond barrier layer will typically have a different composition thanthat of barrier layer 328, and performs essentially the same function ofpreventing threading dislocations from propagating.

A window layer 331 preferably composed of n+ type GaInP is thendeposited over the second barrier layer 330, if there is one disposedover layer 329, or directly over second metamorphic layer 329. Thiswindow layer operates to reduce the recombination loss in the fifthsubcell “E”. It should be apparent to one skilled in the art thatadditional layers may be added or deleted in the cell structure withoutdeparting from the scope of the present invention.

On top of the window layer 331, the layers of cell E are deposited: then+ emitter layer 332, and the p-type base layer 333. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers332 and 333 will be discussed in connection with FIG. 20B.

A BSF layer 334, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell D, the BSF layer performing the samefunction as the BSF layers 308, 313, 318, and 326.

The p++/n++ tunnel diode layers 335 a and 335 b respectively aredeposited over the BSF layer 334, similar to the layers 309 a/309 b, and319 a/319 b, forming an ohmic circuit element to connect subcell E tosubcell F. The layer 335 a is preferably composed of p++ AlGaInAs, andlayer 335 b is preferably composed of n++ GaInP.

In some embodiments a barrier layer 336, preferably composed of n-typeGaInP, is deposited over the tunnel diode 335 a/335 b, to a thickness ofabout 0.5 micron. Such bather layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells D and E, or in the direction of growthinto the subcell F, and is more particularly described in copending U.S.patent application Ser. No. 11/860,183, filed Sep. 24, 2007.

A third metamorphic layer (or graded interlayer) 337 is deposited overthe bather layer 336. Layer 337 is preferably a compositionallystep-graded series of AlGaInAs layers, preferably with monotonicallychanging lattice constant, so as to achieve a gradual transition inlattice constant in the semiconductor structure from subcell D tosubcell E while minimizing threading dislocations from occurring. Insome embodiments the band gap of layer 258 is constant throughout itsthickness, preferably approximately equal to 1.1 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell D. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.1 eV or other appropriate band gap.

In one embodiment of the present disclosure, an optional second barrierlayer (not shown) may be deposited over the AlGaInAs metamorphic layer337. The second bather layer will typically be a different alloy thanthat of barrier layer 336, and performs essentially the same function ofpreventing threading dislocations from propagating.

A window layer 338 preferably composed of n+ type AlGaInAs is thendeposited over the second bather layer, if there is one disposed overlayer 337, or directly over second metamorphic layer 337. This windowlayer operates to reduce the recombination loss in the sixth subcell“F”. It should be apparent to one skilled in the art that additionallayers may be added or deleted in the cell structure without departingfrom the scope of the present invention.

On top of the window layer 338, the layers of cell F are deposited: then+ emitter layer 339, and the p-type base layer 340. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers339 and 340 will be discussed in connection with FIG. 20B.

A BSF layer 341, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell F, the BSF layer performing the samefunction as the BSF layers 308, 313, 326, and 334.

Finally a high band gap contact layer 342, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 341.

The composition of this contact layer 342 located at the bottom(non-illuminated) side of the lowest band gap photovoltaic cell (i.e.,subcell “F” in the depicted embodiment) in a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (i) the backside ohmic metal contact layerbelow it (on the non-illuminated side) will also act as a mirror layer,and (ii) the contact layer doesn't have to be selectively etched off, toprevent absorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

The subsequent remaining steps in the fabrication of the multijunctionsolar cell according to the illustrated embodiment, including thedeposition of metallization over the contact layer, and the attachmentof a surrogate substrate, will be discussed and depicted at a laterpoint in connection with FIG. 7 and subsequent Figures.

Five Junction Solar Cell with One Metamorphic Layer

FIG. 5 depicts a multijunction solar cell in an embodiment according tothe present disclosure in which a five junction solar cell with onemetamorphic buffer layer is fabricated.

FIG. 5 depicts the sequential formation of the five subcells A, B, C, D,and E on a GaAs growth substrate. The layers 401 through 423 of thisembodiment are substantially identical to layers 201 through 223discussed in connection with the embodiment of FIG. 3B, and therefore inthe interest of brevity of this disclosure, the description of suchlayers will not be repeated here.

On top of the window layer 423, the layers of cell D are deposited: then+ emitter layer 424, and the p-type base layer 425. These layers arepreferably composed of n+ type GaInP and p type AlGaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers424 and 425 will be discussed in connection with FIG. 20B.

A BSF layer 426, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell D, the BSF layer performing the samefunction as the BSF layers 408, 413 and 418.

The p++/n++ tunnel diode layers 427 a and 427 b respectively aredeposited over the BSF layer 426, similar to the layers 414 a/414 b and419 a/419 b, forming an ohmic circuit element to connect subcell D tosubcell E. The layer 427 a is composed of p AlGaInAs, and layer 427 b iscomposed of n++ AlGaInAs.

A window layer 428 preferably composed of n+ type GaInP is thendeposited over the tunnel diode layers 427 a and 427 b. This windowlayer operates to reduce the recombination loss in the fifth subcell“E”. It should be apparent to one skilled in the art that additionallayers may be added or deleted in the cell structure without departingfrom the scope of the present invention.

On top of the window layer 428, the layers of cell E are deposited: then+ emitter layer 429, and the p-type base layer 430. These layers arepreferably composed of n+ type GaInP and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well. The doping profile of layers429 and 430 will be discussed in connection with FIG. 20B.

A BSF layer 431, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell E, the BSF layer performing the samefunction as the BSF layers 408, 413, 418, and 426.

Finally a high band gap contact layer 432, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 431.

The composition of this contact layer 432 located at the bottom(non-illuminated) side of the lowest band gap photovoltaic cell (i.e.,subcell “E” in the depicted embodiment) in a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (i) the backside ohmic metal contact layerbelow it (on the non-illuminated side) will also act as a mirror layer,and (ii) the contact layer doesn't have to be selectively etched off, toprevent absorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

The subsequent remaining steps in the fabrication of the multijunctionsolar cell according to the illustrated embodiment, including thedeposition of metallization over the contact layer, and the attachmentof a surrogate substrate, will be discussed and depicted at a laterpoint in connection with FIG. 7 and subsequent Figures.

Six Junction Solar Cell with Two Metamorphic Layers

FIG. 6 depicts the sequence of steps in forming a multijunction solarcell in an embodiment according to the present disclosure in which a sixjunction solar cell with two metamorphic buffer layers is fabricated.

FIG. 6 depicts the sequential formation of the six subcells A, B, C, D,E and F on a GaAs growth substrate. The layers 501 through 527 b of thisembodiment are substantially identical to layers 301 through 327 bdiscussed in connection with the embodiment of FIG. 4, and layers 528through 539 of this embodiment are substantially identical to layers 331through 342 discussed in connection with the embodiment of FIG. 4, andtherefore in the interest of brevity of this disclosure, the descriptionof such layers will not be repeated here.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

The subsequent remaining steps in the fabrication of the multijunctionsolar cell according to the illustrated embodiment, including thedeposition of metallization over the contact layer, and the attachmentof a surrogate substrate, will be discussed and depicted at a laterpoint in connection with FIG. 7 and subsequent Figures.

FIG. 7 is a simplified cross-sectional view of the solar cell of eitherFIG. 2H, 3B, 3C, 4, 5, or 6 depicting just a few of the top layers andlower layers after the next sequence of process steps in which ametallization layer 130 is deposited over the p type contact layer and asurrogate substrate 132 attached using an adhesive or other type ofbonding material or layer 131.

FIG. 8 is a cross-sectional view of the solar cell of FIG. 7 after thenext sequence of process steps in which the growth substrate 101 isremoved.

FIG. 9 is a another cross-sectional view of the solar cell of FIG. 8,but here oriented and depicted with the surrogate substrate 132 at thebottom of the figure.

FIG. 10 is a cross-sectional view of the solar cell of FIG. 9 after thenext process step in which the buffer layer 102, and the etch stop layer103 is removed by a HCl/H₂O solution.

FIG. 11 is a cross-sectional view of the solar cell of FIG. 10 after thenext sequence of process steps in which a photoresist mask (not shown)is placed over the contact layer 104 to form the grid lines 601. As willbe described in greater detail below, a photoresist layer is depositedover the contact layer 104, and lithographically patterned with thedesired grid pattern. A metal layer is then deposited over the patternedphotoresist by evaporation. The photoresist mask is then subsequentlylifted off, leaving the finished metal grid lines 601 as depicted in theFigures.

As more fully described in U.S. patent application Ser. No. 12/218,582filed Jul. 18, 2008, hereby incorporated by reference, the grid lines601 are preferably composed of a sequence of layers Pd/Ge/Ti/Pd/Au,although other suitable materials and layered sequences may be used aswell.

FIG. 12 is a cross-sectional view of the solar cell of FIG. 11 after thenext process step in which the grid lines are used as a mask to etchdown the surface to the window layer 105 using a citric acid/peroxideetching mixture.

FIG. 13A is a top plan view of a wafer 600 according to the presentdisclosure in which four solar cells are implemented. The depiction offour cells is for illustration purposes only, and the present disclosureis not limited to any specific number of cells per wafer.

In each cell there are grid lines 601 (more particularly shown incross-section in FIG. 12), an interconnecting bus line 602, and acontact pad 603. The geometry and number of grid and bus lines and thecontact pad are illustrative and the present disclosure is not limitedto the illustrated embodiment.

FIG. 13B is a bottom plan view of the wafer according to the presentdisclosure with four solar cells shown in FIG. 13A.

FIG. 14 is a cross-sectional view of the solar cell of FIG. 12 after thenext process step in which an antireflective (ARC) dielectric coatinglayer 140 is applied over the entire surface of the “top” side of thewafer over the grid lines 601.

FIG. 15 is a cross-sectional view of the solar cell of FIG. 14 after thenext process step in one embodiment according to the present disclosurein which first and second annular channels 610 and 611, or portion ofthe semiconductor structure are etched down to the metal layer 130 usingphosphide and arsenide etchants. These channels define a peripheralboundary between the cell and the rest of the wafer, and leave a mesastructure which constitutes the solar cell. The cross-section depictedin FIG. 15 is that as seen from the A-A plane shown in FIG. 16. In oneembodiment, channel 610 is substantially wider than that of channel 611.

FIG. 17 is a cross-sectional view of the solar cell of FIG. 15 after thenext process step in another embodiment of the present disclosure inwhich a cover glass 614 is secured to the top of the cell by an adhesive613. The cover glass 614 preferably covers the entire channel 610, butdoes not extend to the periphery of the cell near the channel 611.Although the use of a cover glass is one embodiment, it is not necessaryfor all implementations, and additional layers or structures may also beutilized for providing additional support or environmental protection tothe solar cell.

FIG. 18 is a cross-sectional view of the solar cell of FIG. 17 after thenext process step of the present disclosure in an embodiment in whichthe bond layer 131, the surrogate substrate 132 and the peripheralportion 612 of the wafer is entirely removed, breaking off in the regionof the channel 611, leaving only the solar cell with the cover glass 614(or other supporting layers or structures) on the top, and the metalcontact layer 130 on the bottom. The metal contact layer 130 forms thebackside contact of the solar cell. The surrogate substrate is removedby the use of the Wafer Bond solvent, or other techniques. As notedabove, the surrogate substrate includes perforations over its surfacethat allow the flow of solvent through the surrogate substrate 132 topermit its lift off. The surrogate substrate may be reused in subsequentwafer processing operations.

FIG. 19 is a cross-sectional view of the solar cell of FIG. 18 after thenext sequence of process steps in an embodiment in which the solar cellis attached to a support. In some embodiments, the support may be a thinmetallic flexible film 140. More particularly, in such embodiments, themetal contact layer 130 may be attached to the flexible film 140 by anadhesive (either metallic or non-metallic), or by metal sputteringevaporation, or soldering. In one embodiment, the thin film 140 may beKapton™ or another suitable polyimide material which has a metalliclayer on the surface adjoining the metal contact layer 130. Referencemay be made to U.S. patent application Ser. No. 11/860,142 filed Sep.24, 2007, depicting utilization of a portion of the metal contact layer130 as a contact pad for making electrical contact to an adjacent solarcell.

One aspect of some implementations of the present disclosure, such asdescribed in U.S. patent application Ser. No. 12/637,241, filed Dec. 14,2009, is that the metallic flexible film 140 has a predeterminedcoefficient of thermal expansion, and the coefficient of thermalexpansion of the semiconductor body closely matches the predeterminedcoefficient of thermal expansion of the metallic film 140. Moreparticularly, in some embodiments the coefficient of thermal expansionof the metallic film that has a value within 50% of the coefficient ofthermal expansion of the adjacent semiconductor material.

In some implementations, the metallic film 141 is a solid metallic foil.In other implementations, the metallic film 141 comprises a metalliclayer deposited on a surface of a Kapton or polyimide material. In someimplementations, the metallic layer is composed of copper.

In some implementations, the semiconductor solar cell has a thickness ofless than 50 microns, and the metallic flexible film 141 has a thicknessof approximately 75 microns.

In some implementations, the metal electrode layer may have acoefficient of thermal expansion within a range of 0 to 10 ppm perdegree Kelvin different from that of the adjacent semiconductor materialof the semiconductor solar cell. The coefficient of thermal expansion ofthe metal electrode layer may be in the range of 5 to 7 ppm per degreeKelvin.

In some implementations, the metallic flexible film comprisesmolybdenum, and in some implementations, the metal electrode layerincludes molybdenum.

In some implementations, the metal electrode layer includes aMo/Ti/Ag/Au, Ti/Mo/Ti/Ag, or Ti/Au/Mo sequence of layers.

FIG. 20A is a graph of a doping profile in the emitter and base layersin the top subcell “A” of the inverted metamorphic multijunction solarcell of the present disclosure. As noted in the description of FIG. 3A,the emitter of the upper first solar subcell is composed of a firstregion 206 a in which the doping is graded from 3×10¹⁸ to 1×10¹⁸ freecarriers per cubic centimeter, and a second region 206 b directlydisposed over the first region in which the doping is constant at 1×10¹⁷free carriers per cubic centimeter. Adjacent to the second region 206 bis a the first surface of a spacer region 206 c, and adjacent to thesecond surface of the spacer region is the base layer 108 a.

The specific doping profiles depicted herein (e.g., a linear profile)are merely illustrative, and other more complex profiles may be utilizedas would be apparent to those skilled in the art without departing fromthe scope of the present disclosure.

FIG. 20B is a graph of a doping profile in the emitter and base layersin one or more of the other subcells (i.e., other than the top subcell)of the inverted metamorphic multijunction solar cell of the presentdisclosure. The various doping profiles within the scope of the presentdisclosure, and the advantages of such doping profiles are moreparticularly described in copending U.S. patent application Ser. No.11/956,069 filed Dec. 13, 2007, herein incorporated by reference. Thedoping profiles depicted herein are merely illustrative, and other morecomplex profiles may be utilized as would be apparent to those skilledin the art without departing from the scope of the present disclosure.

FIG. 21 is a graph representing the Al, Ga and In mole fractions versusthe Al to In mole fraction in a AlGaInAs material system that isnecessary to achieve a constant 1.5 eV band gap.

FIG. 22 is a diagram representing the relative concentration of Al, In,and Ga in an AlGaInAs material system needed to have a constant band gapwith various designated values (ranging from 0.4 eV to 2.1 eV) asrepresented by curves on the diagram. The range of band gaps of variousGaInAlAs materials is represented as a function of the relativeconcentration of Al, In, and Ga. This diagram illustrates how theselection of a constant band gap sequence of layers of GaInAlAs used inthe metamorphic layer may be designed through the appropriate selectionof the relative concentration of Al, In, and Ga to meet the differentlattice constant requirements for each successive layer. Thus, whether1.5 eV or 1.1 eV or other band gap value is the desired constant bandgap, the diagram illustrates a continuous curve for each band gap,representing the incremental changes in constituent proportions as thelattice constant changes, in order for the layer to have the requiredband gap and lattice constant.

FIG. 23 is a graph that further illustrates the selection of a constantband gap sequence of layers of GaInAlAs used in the metamorphic layer byrepresenting the Ga mole fraction versus the Al to In mole fraction inGaInAlAs materials that is necessary to achieve a constant 1.51 eV bandgap.

FIG. 24 is a graph that depicts the current and voltage characteristicsof two test multijunction solar cells fabricated according to thepresent disclosure. The current and voltage characteristics of the firsttest solar cell, is shown in solid lines. In the first test cell, theemitter of the top solar subcell is composed of a first region in whichthe doping is graded from 3×10¹⁸ to 1×10¹⁸ free carriers per cubiccentimeter, and a second region directly disposed over the first regionin which the doping is constant at 1×10¹⁷ free carriers per cubiccentimeter, as depicted in the solar cell of FIG. 3A. The current andvoltage characteristics of the second test solar cell, is shown indashed lines ( - - - - - - ) In the second test solar cell, the emitterof the top solar subcell is composed of a first region in which thedoping is graded from 3 to 1×10¹⁸/cm³ and a second region directlydisposed over the first region in which the doping is constant at1×10¹⁸/cm³, as is representative of the solar cell depicted in FIG. 2Aand others described in the parent U.S. patent application Ser. No.12/271,192 filed Nov. 14, 2008.

FIG. 25 is a graph that depicts the measured external quantum efficiency(EQE) as a function of wavelength of the two test multijunction solarcells noted in FIG. 24 above. The external quantum efficiency of thefirst test solar cell, is shown in a dashed line. The external quantumefficiency of the second test solar cell, is shown in a solid line( - - - - - ). A comparison of the external quantum efficiency (EQE)measurements shown in FIG. 25 indicate that the EQE was substantiallyhigher in the wavelength range from 350 nm to 600 nm for the first testsolar cell compared with that of the second test solar cell.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

Although described embodiments of the present disclosure utilizes avertical stack of four, five, or six subcells, various aspects andfeatures of the present disclosure can apply to stacks with fewer orgreater number of subcells, i.e. two junction cells, three junctioncells, seven junction cells, etc. In the case of seven or more junctioncells, the use of more than two metamorphic grading interlayer may alsobe utilized.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell A, with p-type and n-type GaInP is one example of ahomojunction subcell. Alternatively, as more particularly described inU.S. patent application Ser. No. 12/023,772 filed Jan. 31, 2008, thesolar cell of the present disclosure may utilize one or more, or all,heterojunction cells or subcells, i.e., a cell or subcell in which thep-n junction is formed between a p-type semiconductor and an n-typesemiconductor having different chemical compositions of thesemiconductor material in the n-type regions, and/or different band gapenergies in the p-type regions, in addition to utilizing differentdopant species and type in the p-type and n-type regions that form thep-n junction.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or the emitter layer may alsobe intrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP,AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs,GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AIN, GaN, InN,GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials,and still fall within the spirit of the present invention.

While the solar cell described in the present disclosure has beenillustrated and described as embodied in an inverted metamorphicmultijunction solar cell, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the foregoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

The invention claimed is:
 1. A multijunction solar cell having at least four solar subcells, the multijunction solar cell comprising: a first solar subcell having a first band gap; a first graded interlayer adjacent to the first solar subcell, said first graded interlayer having a second band gap greater than said first band gap and that is constant at 1.5 eV throughout the thickness of the first graded interlayer; and a second solar subcell adjacent to said first graded interlayer, said second solar subcell having a third band gap smaller than the first band gap of the first solar subcell wherein said second solar subcell is lattice mismatched with respect to said first solar subcell; a second graded interlayer adjacent to said second solar subcell, said second graded interlayer having a fourth band gap greater than said third band gap of the second solar subcell and that is constant at 1.1 eV throughout the thickness of the second graded interlayer; and a third solar subcell adjacent to said second graded interlayer, said third solar subcell having a fifth band gap smaller than said third band gap of the second solar subcell wherein said third solar subcell is lattice mismatched with respect to said second solar subcell, wherein each of the first and second graded interlayers is composed, respectively, of a compositionally step-graded series of (In_(x)Ga_(1-x))_(y)Al_(1-y)As layers with monotonically changing lattice constant, with x and y having respective values such that the band gap of each interlayer remains constant throughout its thickness, and wherein 0<x<1 and 0<y<1.
 2. The multijunction solar cell of claim 1 further comprising a barrier layer adjacent at least one of the first or second graded interlayers.
 3. The multijunction solar cell of claim 2 wherein the barrier layer is composed of (Al)GaInP.
 4. The multijunction solar cell of claim 2 wherein the barrier layer is disposed between the first graded interlayer and the second solar subcell.
 5. The multijunction solar cell of claim 2 wherein the barrier layer is disposed between the second solar subcell and second graded interlayer.
 6. The multijunction solar cell of claim 1 wherein the first graded interlayer is disposed between first and second barrier layers that are adjacent to the first graded interlayer.
 7. The multijunction solar cell of claim 6 wherein the first barrier layer has a composition different from a composition of the second barrier layer.
 8. The multijunction solar cell of claim 1 wherein the second graded interlayer is disposed between first and second barrier layers that are adjacent to the second graded interlayer.
 9. The multijunction solar cell of claim 8 wherein the first barrier layer has a different composition from the second barrier layer.
 10. The multijunction solar cell of claim 1 wherein: the compositionally step-graded series of (In_(x)Ga_(1-x))_(y)Al_(1-y)As layers in the first graded interlayer provide a gradual transition in lattice constant from the first solar subcell to the second solar subcell, and the compositionally step-graded series of (In_(x)Ga_(1-x))_(y)Al_(1-y)As layers in the second graded interlayer provide a gradual transition in lattice constant from the second solar subcell to the third solar subcell.
 11. A multijunction solar cell having at least four solar subcells, the multijunction solar cell comprising: a first solar subcell having a first band gap; a first graded interlayer and a second graded interlayer, and a plurality of additional solar subcells stacked one over the other and disposed between the first and second graded interlayers, the additional solar subcells including a second solar subcell and a third solar subcell, the first graded interlayer being adjacent to the first solar subcell, said first graded interlayer having a second band gap greater than said first band gap of the first solar subcell and that is constant at 1.5 eV throughout the thickness of the first graded interlayer; and the second solar subcell being adjacent to said first graded interlayer, said second solar subcell having a third band gap smaller than the first band gap of the first solar subcell wherein said second solar subcell is lattice mismatched with respect to said first solar subcell; the third solar subcell being adjacent to a first side of said second graded interlayer, said third solar subcell having a fourth band gap smaller than the third band gap of the second solar subcell; the second graded interlayer having a fifth band gap greater than said fourth band gap of the third solar subcell and that is constant at 1.1 eV throughout the thickness of the second graded interlayer; and the multijunction solar cell further including a fourth solar subcell adjacent to a second side of said second graded interlayer, said fourth solar subcell having a sixth band gap smaller than said fifth band gap of the third solar subcell wherein said fourth solar subcell is lattice mismatched with respect to said third solar subcell, wherein each of the first and second graded interlayers is composed, respectively, of a compositionally step-graded series of (In_(x)Ga_(1-x))_(y)Al_(1-y)As layers with monotonically changing lattice constant, with x and y having respective values such that the band gap of each interlayer remains constant throughout its thickness, and wherein 0<x<1 and 0<y<1.
 12. The multijunction solar cell of claim 11 further comprising a barrier layer adjacent at least one of the first or second graded interlayers.
 13. The multijunction solar cell of claim 12 wherein the barrier layer is composed of (Al)GaInP.
 14. The multijunction solar cell of claim 11 wherein the barrier layer is disposed between the first graded interlayer and the second solar subcell.
 15. The multijunction solar cell of claim 11 wherein the barrier layer is disposed between the third solar subcell and second graded interlayer.
 16. The multijunction solar cell of claim 11 wherein the first graded interlayer is disposed between first and second barrier layers that are adjacent to the first graded interlayer.
 17. The multijunction solar cell of claim 16 wherein the first barrier layer has a composition different from a composition of the second barrier layer.
 18. The multijunction solar cell of claim 11 wherein the barrier layer is adjacent the second graded interlayer.
 19. The multijunction solar cell of claim 11 wherein: the compositionally step-graded series of (In_(x)Ga_(1-x))_(y)Al_(1-y)As layers in the first graded interlayer provide a gradual transition in lattice constant from the first solar subcell to the second solar subcell, and the compositionally step-graded series of (In_(x)Ga_(1-x))_(y)Al_(1-y)As layers in the second graded interlayer provide a gradual transition in lattice constant from the second solar subcell to the third solar subcell.
 20. The multijunction solar cell of claim 1 further comprising: a fourth solar subcell adjacent the first solar subcell, the fourth solar subcell being disposed on a side of the first solar subcell opposite a side of the first solar subcell on which the first graded interlayer is disposed, wherein the fourth solar subcell has a sixth band gap greater than the first band gap of the first solar subcell; and a fifth solar subcell adjacent the fourth solar subcell, the fifth solar subcell being disposed on a side of the fourth solar subcell opposite a side of the fourth solar subcell on which the first solar subcell is disposed, wherein the fifth solar subcell has a seventh band gap greater than the sixth band gap of the fourth solar subcell, and wherein the fifth solar subcell is positioned as a top solar subcell of the multijunction solar cell that faces solar radiation during operation.
 21. The multijunction solar cell of claim 11 further comprising: a fifth solar subcell adjacent the first solar subcell, the fifth solar subcell being disposed on a side of the first solar subcell opposite a side of the first solar subcell on which the first graded interlayer is disposed, wherein the fifth solar subcell has a seventh band gap greater than the first band gap of the first solar subcell; and a sixth solar subcell adjacent the fifth solar subcell, the sixth solar subcell being disposed on a side of the fifth solar subcell opposite a side of the fifth solar subcell on which the first solar subcell is disposed, wherein the sixth solar subcell has an eighth band gap greater than the seventh band gap of the fifth solar subcell, and wherein the sixth solar subcell is positioned as a top solar subcell of the multijunction solar cell that faces solar radiation during operation. 